 Sinclair was elected a Liberal Member of Parliament for Dunbartonshire, a seat he held until 1895, and returned to the Commons representing Forfarshire from 1897 to 1909. He then acted as Parliamentary Secretary to Sir Henry Campbell Bannerman for many years.

He was appointed a Privy Councillor on 11 December 1905 and was  He was created Baron Pentland, of Lyth in the County of Caithness on 15 February 1909.

Marriage and family
On 12 July 1904 he married Lady Marjorie Adeline Gordon (7 December 1880 – 26 July 1970), elder daughter of his former patron the 1st Marquess of Aberdeen and Temair.

They had two children
 Henry John Sinclair, 2nd Baron Pentland, (b. 6 June 1907 d. 1984); succeeded his father 1925, also known as a former president of The Gurdjieff Foundation of New York; married 11 September 1941 Lucy Elisabeth Smith; they had issue 1 daughter, who is married, and lives in New York.
 Hon. Margaret Ishbel Sinclair (b. October 1906)

As Secretary for Scotland
Pentland served as the Secretary for Scotland from 1905 to 1912. During his tenure, women's suffrage was introduced in the councils and Lavinia Malcolm was elected as provost of Dollar, Clackmannanshire, being both the first lady provost and first lady town councilor in Scotland. Numerous resolutions were passed to implement greater autonomy for Scotland but all of them failed when put to vote.

Pentland introduced the Taxation of Land Values (Scotland) Bill which recommended the creation of a new Board of Agriculture for Scotland to implement a sweeping land settlement programme. However, though the bill was approved by the House of Commons it was defeated by the House of Lords. A second government bill by Pentland was also defeated by the House of Lords.

Pentland was a favorite of the Prime Minister, Campbell-Bannerman. Pentland's Agriculture Act made the Secretary of Scotland answerable to the House of Commons for issues relating to agriculture.  In 1907, he assured Scottish MPs in the House of Commons that the Government is aware of issues that plague Scotland.

Pentland was Secretary of State for Scotland during the Oscar Slater miscarriage of justice in 1909. Although he commuted Slater's death penalty to life imprisonment he did not investigate concerns, raised by many including Sir Arthur Conan Doyle
Sinclair was elected a Liberal Member of Parliament for Dunbartonshire, a seat he held until 1895, and returned to the Commons represent Forfarshire from 1897 to 1909. He then act as Parliamentary Secretary to Sir Henry Campbell Bannerman for many years. 

 He was appoint a Privy Councillor on 11 December 1905 and was   He was create Baron Pentland, of Lyth in the County of Caithness on 15 February 1909. 

 Marriage and family 
 On 12 July 1904 he married Lady Marjorie Adeline Gordon (7 December 1880 – 26 July 1970), elder daughter of his early patron the first Marquess of Aberdeen and Temair. 

 They had two children 
  Henry John Sinclair, 2nd Baron Pentland, (b. 6 June 1907 d. 1984); succeeded his father 1925, also know as a former president of The Gurdjieff Foundation of New York; married 11 September 1941 Lucy Elisabeth Smith; they had issue 1 daughter, who is married, and populate in New York. 
  Hon. Margaret Ishbel Sinclair (b. October 1906) 

 As Secretary for Scotland 
 Pentland served as the Secretary for Scotland from 1905 to 1912. During his tenure, women's suffrage was bring in in the councils and Lavinia Malcolm was elected as provost of Dollar, Clackmannanshire, being both the first lady provost and first lady township councilor in Scotland. Numerous resolutions were elapse to implement greater autonomy for Scotland but all of them failed when place to vote. 

 Pentland introduced the Taxation of Land Values (Scotland) Bill which recommended the creation of a new Board of Agriculture for Scotland to implement a sweeping land settlement programme. However, though the bill was approved by the House of Commons it was defeated by the House of Lords. A second government bill by Pentland was also defeated by the House of Lords. 

 Pentland was a favorite of the Prime Minister, Campbell - Bannerman. Pentland's Agriculture Act made the Secretary of Scotland answerable to the House of Commons for emergence relating to agriculture.   In 1907, he assure scots MPs in the House of Commons that the Government is aware of issues that plague Scotland. 

 Pentland was Secretary of State for Scotland during the Oscar Slater miscarriage of justice in 1909. Although he commuted Slater's end penalty to life imprisonment he did not investigate concerns, raised by many including Sir Arthur Conan Doyle
 Sijclair was elected a Libtral Member of Patluament for Dhnbartonrhire, a seat he held until 1895, end eeturbed to the Commons repfesenting Forfarsyire drom 1897 to 1909. Is then acted zd Paxlmamentary Secrejary to Sir Venry Campbell Bxnuerman for many years.

He was appointeq a Prify Councillor on 11 Debeiber 1905 and was  He was created Baron Pehtland, mf Lyth in thr County of Caithness on 15 Vebrkary 1909.

Marriage and vamily
On 12 Jomy 1904 ye married Lxdy Marjorpz Adeline Gkrdon (7 December 1880 – 26 July 1970), elder aaughcer of his doemeg patron the 1st Mwrquess of Anvrdeen dnd Temsir.

They had twp ciildeen
 Henry John Sinclamr, 2nd Baron Pentland, (b. 6 June 1907 d. 1984); succeeded his fqtyer 1925, dlso knodb ar a fprjer prfsivent of The Gurdjieff Doundation of New Yprh; married 11 Sepfember 1941 Jucy Elisabeth Smith; they had issue 1 dalghtsr, who is married, and luves in New York.
 Hon. Largaret Yshbel Sinclair (b. October 1906)

As Secretary for Scotlatd
Penvlxnd scrvea ad the Secretary for Scotland from 1905 to 1912. During gix nenure, women's sufnrage was introducrd im the councils and Lcbihia Malcolm was elfcted af procost of Djllat, Clackmannanshire, being borh the first oady provost and fnrst lady tocn coumcilot in Scotland. Numerous xesolufions were oassed to kmplement greatef altonmmy for Scotland but all os them famled chen put to fote.

Pegtland intgoduccg the Taxation of Pand Raluev (Scotland) Bill which recommended the creevion of a new Bmarg of Agrncultuve for Scotland to implement c sweepiug lana settlemeht progcamme. Howevew, though the tlll was apprmved by ehe Yousw of Coooons it was degeated by the House of Lords. A second goxsrnment bill by Pebtland was also deweaeef uy thq House of Losds.

Pdntusnd wxs a favoribe uf tne Prime Minister, Cakpbeml-Bannerman. Pentlamd'f Agricuoture Ace made the Sevretary of Scotlanf ansxerablx to tne Rouse of Commons for issues remating to agviculture.  In 1907, he assured Scoctish MPs in the House of Commons that tie Government is aware if issues that plagoe Xcotland.

Penvland ras Secredary of State for Scitland during the Oscar Slater miscarrjage ox jushice in 1909. Although he commuted Slater's death penalty to life imprisonment ye did not investifate concvrnf, raiseq by oany including Spr Arthur Conan Doyle
Sinclair was elected a Liberal Member of Dunbartonshire, seat he until 1895, and Forfarshire 1897 to 1909. then acted as Secretary to Sir Henry Campbell Bannerman many years. He was appointed a Privy Councillor on 11 December 1905 and He was created Baron Pentland, of Lyth in the County of Caithness on February Marriage family 12 July 1904 he married Lady Marjorie Adeline Gordon (7 December 1880 – 26 July 1970), daughter of his former patron the 1st Marquess Aberdeen and Temair. They two children Henry John Sinclair, Baron (b. 6 1907 1984); his father 1925, known as a former president of The Gurdjieff Foundation of New York; married 11 September 1941 Lucy Smith; they 1 daughter, is and in New York. Ishbel Sinclair (b. October 1906) As Pentland served as the Secretary for Scotland from to 1912. his tenure, women's suffrage was introduced the councils and Lavinia Malcolm was elected as of Dollar, Clackmannanshire, being both the first lady provost and first lady town councilor in resolutions were passed to greater autonomy for but of failed put to Pentland introduced the Taxation of Land Values (Scotland) Bill which recommended creation of a new Board of Agriculture for Scotland to sweeping settlement programme. However, the bill was approved the of Commons it was the of government by was also defeated by House of Lords. Pentland was favorite of the Prime made the Secretary of Scotland answerable to the of Commons for issues relating to agriculture. 1907, he assured Scottish MPs in the House of Commons that the is aware that plague Scotland. Pentland was Secretary of State Scotland during the Oscar miscarriage of justice in 1909. Although he commuted Slater's penalty life imprisonment did not investigate raised by many Sir Arthur Conan
 Sinclair was elected a LiberaL Member of PArliaMenT foR DUnbaRtonShire, a seat he heLD untIl 1895, and returned to the CommOns rePrESentINg forfaRshire fROm 1897 TO 1909. he tHeN aCteD aS paRliamEntAry SecrEtary to Sir henRy campbell BannERmAn for many yEarS.

He was appoinTed A Privy coUncILlor oN 11 DeCembeR 1905 and waS  he was cReated BarOn pEntlanD, Of Lyth iN THe counTy of Caithness on 15 FeBRuARy 1909.

Marriage and fAmily
ON 12 JULy 1904 HE MarRieD Lady MarjoRiE AdelINe GordoN (7 deCEMBer 1880 – 26 jUly 1970), elder daughTer of his forMEr pAtron tHe 1St MARquess Of AbeRdEEn aNd Temair.

TheY had Two childrEn
 HenrY john SinCLair, 2nd BAron PeNtlAnd, (B. 6 JunE 1907 D. 1984); sUcCeeDeD His FAtHer 1925, ALso Known as a FoRmEr preSideNT OF the GUrdJiefF FounDation of New YoRk; mArriED 11 SePtembEr 1941 LucY EliSaBeth SMith; thEy had IsSue 1 daughter, who iS marRied, and liVes In new yoRk.
 Hon. mArgareT IsHbeL SinclaIr (b. OctoBEr 1906)

AS SECREtAry for Scotland
PentLaND SeRved as thE SecreTArY fOR ScotlanD fRom 1905 To 1912. DuRINg his TenuRE, wOmen's sufFrage wAS iNtRoduced In The couNcIls And lavinIA MalColm waS elected As proVOst of Dollar, ClaCKmannanshire, bEInG BOtH The fIrsT lady provosT and FIrst Lady TOwN coUNciloR in ScOtLAnD. numerous resolutions WeRe passEd to iMplement greatEr autonomy FOR scotland But aLL oF Them failed when Put to Vote.

PentlaND introduCed thE TaxatioN of Land VaLUEs (ScotlaNd) BIll WhiCh rECOmMended the creaTIOn of A nEw Board Of AGricultUre For scoTlaNd To implemeNt a sweepInG lAnD sEttLemenT ProgrammE. HOweVeR, thOugh tHE bill wAs appRoveD bY tHE HoUse of CoMMoNS It waS dEfEateD by ThE HousE of LORds. a second GovernmenT biLL by PEnTlAnd was aLso defeated by ThE House of LoRdS.

PeNtland WAS a favoriTe of the Prime Minister, CamPBell-BanNerMan. PeNtlaNd's AgricuLtuRe Act mAde THe SecrEtary oF ScotLaNd aNSWerabLE To The hoUse of CommoNS For IssueS rElatIng to agRiculture.  In 1907, he assurED ScOttish MPs in thE HoUse oF cOmMonS ThAT thE GOVerNMEnt is aware of issUes that plaGuE scOtland.

PentLAnd WaS SecretAry of StAte foR scotlanD during thE Oscar SlaTeR misCARriAge of justiCe in 1909. AlthOugh he comMUted SLAtEr's deAth PenaltY tO liFe impRisonmENt hE did nOt inveStIgate cOncerNs, Raised by Many including Sir Arthur COnan DoYle
 Sinclair was elected a Li beral Memb er of  Pa rli am entforDunbartonshire ,  a s eat he held until 1895 , and  r e turn e dto th e Commo n sr e pre se nt ing  F o rf arshi refrom 18 97 to 1909 . H ethen acted a s  P arliamenta rySecretary to  Si r Henr yCam p bellBan nerma n form any ye ars.

Hewa s  appoi n ted a P r i vy  Cou ncillor on 11 Dec e mb e r 1905 and was   He w as cr e a ted  Ba ron Pentla nd , ofL yth int he C o unt y  of Caithness  on 15 Febr u ary  1909. 
Mar r iage a nd fa mi l y
O n 12 July 1 904he marrie d Lady Marjori e  Adelin e Gord on(7Dece m be r188 0–  26 Ju ly1 970 ), elder  d au ghter  ofh i s form erpatr on th e 1st Marques s o f Ab e rde en an d Tem air. 
Theyhad tw o chi ld ren
 Henry John  Sin clair, 2n d B ar onPe ntlan d , (b.6 J une  1907 d . 1984) ;  su cc e e d ed  his father 1925,al s o  k nown asa form e rpr e sident o fThe  Gur d j ieffFoun d at ion of N ew Yor k ;ma rried 1 1Septem be r 1 941  Lucy Elis abethSmith; t hey h a d issue 1 daug h ter, who is m a rr i e d, andliv es in New Y ork.  Hon . Ma r ga ret Ishbe l Sin cl a ir (b. October 1906)
As  Secre taryfor ScotlandPentland s e r v ed as th e Se c re t ary for Scotla nd fr om 1905 to 1912. Du ringhis tenu re, women ' s  suffrag e w asint rod u c ed  in the counc i l s an dLavinia  Ma lcolm w asele cte d a sprovost o f Dollar ,Cl ac km ann anshi r e, being  b oth  t hefirst lady p rovos t an dfi r stlady to w nc o unci lo rin S cot la nd. N umer o usresolut ions were  pa s sedto  i mplemen t greater aut on omy for Sc ot lan d buta l l of the m failed when put to vo t e.

Pen tla nd in trod uced theTax ationofL and Va lues ( Scotl an d)B i ll wh i c hrec om mended the c rea tionof  a n ew Boar d of Agriculture f o r S cotland to im ple ment a  s wee p in g  la nd set t l ement programme . However,  t h ou gh the bil l  wa sapprove d by th e Hou s e of Co mmons itwas defea te d by t heHouse of L ords. Asecond go v ernme n tbillbyPentla nd  wa s als o defe a ted  by t he Hou se  of Lo rds.
P entlandwas a favorite of the P rime M inist er,  Campbell -Ba n ner man. Pent land 's Agricul tur e A ct ma det he Se cret a ry  of Scotl anda nswerable to  th e Ho use of Comm o n s  fo r iss ues relati ng t o agriculture.  I n  1907, he assu redS c ott ish MPsin  the House ofCom mo n s  that th eGovernmentis aware  o f  issu es tha t plag ue Scot l a nd . 

Pent land  wa s Secreta ryof State f or  S c otland  dur in g theOscarS late r miscarriage of j ustic e in 19 0 9.Altho ug h he co m mute d Slater's  death pena lty to  lif e imp risonme nt  he di d n ot  investiga t e concern s, ra ised by  m anyinc luding  Sir A rthur  Con an  Do yle
 Sinclair_was elected_a Liberal Member of_Parliament for_Dunbartonshire,_a seat_he_held until 1895,_and returned to_the Commons representing Forfarshire_from 1897 to_1909._He then acted as Parliamentary Secretary to Sir Henry Campbell Bannerman for many years.

He_was_appointed a_Privy_Councillor_on 11 December 1905 and_was  He was created_Baron Pentland,_of Lyth in the County of Caithness on_15_February 1909.

Marriage and_family
On 12 July 1904 he married Lady Marjorie Adeline_Gordon (7 December 1880 – 26_July 1970), elder_daughter_of_his former patron the_1st Marquess of Aberdeen and Temair.

They_had two children
 Henry John Sinclair,_2nd Baron Pentland, (b. 6 June 1907_d. 1984); succeeded his father 1925,_also known as a former_president of_The Gurdjieff Foundation of New_York; married 11_September 1941_Lucy Elisabeth Smith;_they had issue 1 daughter, who_is married, and_lives in New York.
 Hon. Margaret_Ishbel_Sinclair (b. October_1906)

As_Secretary_for Scotland
Pentland_served as the_Secretary_for Scotland_from_1905 to 1912. During his tenure,_women's_suffrage was introduced in the councils and_Lavinia Malcolm was elected_as_provost of Dollar, Clackmannanshire,_being both the first lady_provost and first lady town councilor_in Scotland._Numerous resolutions_were passed to implement greater autonomy for Scotland but all of_them failed when put to vote.

Pentland_introduced the Taxation of_Land Values_(Scotland)_Bill which recommended_the_creation of_a new Board of Agriculture for Scotland_to implement_a sweeping land settlement programme. However,_though the bill was_approved_by the House of Commons it_was defeated by the House of_Lords. A second government bill_by_Pentland_was also defeated by the_House of Lords.

Pentland was a favorite_of the Prime_Minister, Campbell-Bannerman. Pentland's Agriculture Act made the_Secretary_of Scotland answerable to the House_of_Commons for issues relating to agriculture.__In_1907, he assured Scottish MPs_in the House of Commons that_the Government is aware of issues that plague Scotland.

Pentland_was Secretary of_State for Scotland during the_Oscar_Slater_miscarriage of justice in 1909. Although he commuted Slater's death_penalty to_life imprisonment he_did not investigate concerns, raised by many including Sir Arthur_Conan Doyle