 As the daughter and only heiress of the eldest son of the 11th Earl of Kildare, the barony of Offaly had been claimed on her behalf when she was a child; in 1599, she assumed the title Baroness Offaly.Lettice has been described as having been an accomplished negotiator, and this skill paid off when finally, on 29 July 1620, after years of dispute, King James I granted her the "suo jure" title of 1st Baroness Offaly for life.This was made under the Great Seal of England, and the King also invested her with the lands of Killeagh, and the territory and demesne of Geashill in King's County, Ireland.