Needing a point to be crowned French champions for the eighth time, the in-form Kylian Mbappe scored the opener in style.
Mbappe, 18, latched on to captain Radamel Falcao's through ball, skipped around the goalkeeper and slotted in.
Forward Valere Germain made sure of the win in injury time with a tap-in from close range.
The home side had all but secured the title before kick-off, with Paris St-Germain three points behind with just one game to play and a far inferior goal difference.
The new champions rarely looked troubled in registering their 11th consecutive league win and subsequently lifting a trophy that has eluded them since the turn of the millennium.
The last time Monaco won Ligue 1 they were fielding a team which included the French trio of Fabien Barthez, David Trezeguet and Willy Sagnol.
Monaco's success this season is largely down to the reinvigorated Colombian Falcao and the emergence of French teenager Mbappe.
Falcao, 31, has put the disappointment of his loan spells with Manchester United and Chelsea, where he scored just five goals in 36 appearances, behind him.
The forward will finish top scorer for Monaco this campaign, netting 24 times with four assists.
Mbappe has caught the eye of suitors from the top teams in Europe in his breakthrough season. He has scored 15 league goals this season to add to his remarkable performances in the Champions League.
Mbappe, voted Ligue 1's young player of the year, scored six in six as Monaco reached the semi-final stage, eventually losing to Juventus over two legs.
That defeat did not overshadow their achievements, however, as Monaco brushed aside Tottenham Hotspur, Manchester City and Borussia Dortmund en route to the semi-finals.
Monaco's triumph is all the more impressive given they finished eighth in Ligue 2, France's second division, five years ago, having been in and out of the relegation zone over the course of the 2011-2012 season.
The following season they finished top of Ligue 2 and returned to top-flight football.
Just two seasons later, they have scored over 104 goals, conceding just 29, on their way to being crowned the best team in France.
Four players from that Ligue 2 winning team are in Monaco's current title-winning squad: Goalkeeper Danijel Subasic, defender Andrea Raggi, winger Nabil Dirar and forward Germain.
Whether they struggle to hold onto the likes of Mbappe awaits to be seen, but the celebrations, prematurely, had already been put in place before Wednesday's victory.
On Wednesday morning, the club announced on Twitter that American rapper 50 Cent would be performing at the players' victory party.
A statement on the club's website read: "The American star will come to celebrate the excellent results of Falcao's team-mates this Sunday in the Palace Square in the Principality."
Before the party can begin, Monaco will end their season with an away trip to ninth placed Rennes on Saturday.
Match ends, Monaco 2, St Etienne 0.
Second Half ends, Monaco 2, St Etienne 0.
Goal!  Monaco 2, St Etienne 0. Valère Germain (Monaco) right footed shot from the centre of the box to the bottom right corner. Assisted by Thomas Lemar following a fast break.
Substitution, Monaco. Andrea Raggi replaces Kylian Mbappe.
Foul by João Moutinho (Monaco).
Jérémy Clément (St Etienne) wins a free kick in the attacking half.
Substitution, Monaco. Guido Carrillo replaces Falcao.
Attempt missed. Kenny Rocha Santos (St Etienne) header from the centre of the box misses to the right. Assisted by Jordan Veretout with a cross following a corner.
Corner,  St Etienne. Conceded by Kamil Glik.
Delay over. They are ready to continue.
Substitution, St Etienne. Kenny Rocha Santos replaces Fabien Lemoine.
Delay in match Benjamin Mendy (Monaco) because of an injury.
Attempt saved. Valère Germain (Monaco) right footed shot from outside the box is saved in the centre of the goal. Assisted by Fabinho.
Delay over. They are ready to continue.
Delay in match Falcao (Monaco) because of an injury.
Leo Lacroix (St Etienne) is shown the yellow card for a bad foul.
Falcao (Monaco) wins a free kick in the defensive half.
Foul by Leo Lacroix (St Etienne).
Attempt missed. Valère Germain (Monaco) right footed shot from the centre of the box is close, but misses to the left. Assisted by Kylian Mbappe.
Attempt missed. Falcao (Monaco) header from the centre of the box misses to the right. Assisted by Djibril Sidibe with a headed pass following a corner.
Corner,  Monaco. Conceded by Kévin Théophile-Catherine.
Substitution, Monaco. Valère Germain replaces Bernardo Silva.
Substitution, St Etienne. Romain Hamouma replaces Alexander Søderlund.
Bernardo Silva (Monaco) is shown the yellow card for a bad foul.
Foul by Bernardo Silva (Monaco).
Jordan Veretout (St Etienne) wins a free kick in the defensive half.
Corner,  Monaco. Conceded by Leo Lacroix.
Corner,  Monaco. Conceded by Leo Lacroix.
Kamil Glik (Monaco) wins a free kick in the defensive half.
Foul by Jordan Veretout (St Etienne).
Attempt saved. Henri Saivet (St Etienne) left footed shot from the centre of the box is saved in the centre of the goal.
Corner,  St Etienne. Conceded by Falcao.
Foul by Djibril Sidibe (Monaco).
Jordan Veretout (St Etienne) wins a free kick on the left wing.
Attempt saved. Arnaud Nordin (St Etienne) left footed shot from the right side of the box is saved in the centre of the goal. Assisted by Kévin Théophile-Catherine.
Attempt blocked. Benjamin Mendy (Monaco) left footed shot from the left side of the box is blocked.
Foul by Fabinho (Monaco).
Henri Saivet (St Etienne) wins a free kick in the defensive half.
Corner,  Monaco. Conceded by Leo Lacroix.
Attempt saved. Falcao (Monaco) header from the centre of the box is saved in the centre of the goal. Assisted by Thomas Lemar with a cross.
Monaco have won the Ligue 1 title for the first time in 17 years after a 2-0 win at home to Saint-Etienne.