Skipper Stephen Craigan scored his first goal in five years in the fifth minute of the semi-final when he capitalised on a blunder by Saints keeper Peter Enckelman to head in a Tom Hateley corner.
Further goals before the break from Jamie Murphy (14) and John Sutton (39) ensured the Steelmen will return to the national stadium on May 21 to face either Aberdeen or Celtic who play at Hampden on Sunday.
There was a disappointingly low attendance of just under 12,000 to see Saints make the better start.
Danny Invincibile's header from a Danny Grainger free-kick in the second minute had Fir Park keeper Darren Randolph stretching to tip the ball over for a corner, which the Lanarkshire club survived.
Three minutes later, however, Motherwell were ahead, thanks to a blunder by Enckelman.
The former Aston Villa keeper flapped at Tom Hateley's corner from the left, allowing Craigan the easiest of tasks to head in from six yards and celebrate his first goal in five years.
Saints were immediately deflated and in the 14th minute their task was made all the more difficult when Murphy drove low past Enckelman after dribbling all too easily to the edge of the box, helped by Saints defenders Steven Anderson and Michael Duberry colliding with each other.
St Johnstone midfielder Murray Davidson flashed a shot from distance just over the Motherwell bar to give Derek McInnes' side some hope.
Then Hateley got his head in the way of Grainger's whipped-in free-kick and was happy to see the ball skip just wide of his own goal and behind for a corner.
However, in the 31st minute Motherwell were almost three ahead when Keith Lasley made a good opening for himself inside the St Johnstone box - but the midfielder dragged his shot just wide of Enckelman's left-hand post.
Duberry was booked for a foul on Lasley before Murphy ended another fine run by driving just wide from inside the box, and moments later he had another effort which went just past the far post.
But in the 38th minute the game was over when Sutton took a pass from Francis Jeffers and arced a shot from 30 yards over the hapless Enckelman and in to the net.
Jordan Robertson replaced Collin Samuel for the start of the second half and four minutes later he was in the book after a late challenge on Motherwell defender Shaun Hutchinson.
In the 51st minute there was a St Johnstone claim for a penalty when defender Dave Mackay's throw-in looked as if it might have come off the top of Lasley's arm in the box - but referee Iain Brines was unimpressed.
Moments later, Grainger had a free-kick from 30 yards gratefully clutched just under his crossbar by Randolph before the keeper made a better save from Davidson's effort from just outside the box.
Robert McHugh came on for Jeffers in the 66th minute while Steven May replaced Kevin Moon for St Johnstone. With Motherwell's cup final appearance secured, boss Stuart McCall replaced Chris Humphrey with Steve Jones.
In the 80th minute Davidson crashed a header off the bar after latching on to a Grainger corner and with that any slim hopes of an unlikely Saints comeback were gone.