The Parkhead side put rumours of their manager, Neil Lennon, replacing David Moyes at Everton to the back of their minds as they claim their second Scottish title in a row
The Glasgow giants will now prepare for a visit to Hampden on May 26, as they take on Hibernian in the Scottish Cup final, looking to make it a double-winning season.
In their final home match of this season, Celtic boss Neil Lennon decided to go with his tried and trusted starting line-up, making six changes, one of which included the return of skipper Scott Brown, who had been out since February.
St Johnstone, meanwhile, made just the one change from the side that defeated Dundee United, with Chris Millar missing out due to injury and Patrick Cregg taking his place.
The home side started the title party in the first minute as Forrest found Ledley on the edge of the area and the Wales international rifled the ball beyond the outstretched arms of Alan Mannus and into the back of the net.
Celtic continued to press forward in search of more goals but found the St Johnstone defence stubborn, with the only other chance of the first half-hour coming the way of Mikael Lustig, though Mannus tipped away the effort for a corner.
A superb ball from Mulgrew almost helped open up the visitors' defence. As he played the ball over the top to Anthony Stokes, the striker could not get a shot off and played it back to Brown, whose strike cannoned off a defender and away to safety.
With 10 minutes left in the half, the SPL champions doubled their advantage when Mulgrew swung in a free kick from the right wing, which evaded everyone, including Mannus, and flew into the far corner of the goal.
Celtic continued to move forward at the start of the second half and added to their lead after Gary Hooper made a run forward after receiving possession on the halfway line and played in Forrest on the right-hand side. The 21-year-old winger made no mistake, firing home from close range.
The home side maintained their dominance over the Perth side during the fixture, with both Forrest and Mulgrew trying their luck, but neither of the Scotland internationals could find the target.
It should have been four with just over 20 minutes remaining as Emilio Izaguirre lofted a cross from the left that was missed by the St Johnstone defence and arrived at the feet of Georgios Samaras but he somehow managed to put his attempt wide from close range.
Seven minutes later, Hoops keeper Fraser Forster was finally called into action as substitute Nigel Hasselbaink made his way into the box but his strike towards goal was tame and allowed the English stopper to save easily.
Lennon's side sealed the win in injury time as Wright headed a corner from the right into his own net to end a miserable day for the Perth side.