Murray Davidson's deflected effort gave the Irn-Bru First Division champions a deserved half-time lead but Shaun Hutchinson headed his first senior goal to level.
Graham Gartland soon scrambled Saints back into the lead but Ross Forbes earned Motherwell a point with a wonderful long-range strike.
Both sides had chances for a winner as they went all out for three points, with McDiarmid Park witnessing a memorable reintroduction to top-flight football following a seven-year absence.
St Johnstone showed plenty of attacking ambition with Filipe Morais starting brightly in Derek McInnes' three-man forward line.
Tom Hateley, who joined Motherwell on Thursday, was a late addition to their starting team after Paul Slane suffered a calf injury during the warm-up. The 19-year-old came in at right-back with Jim O'Brien starting on the left wing in a 4-1-4-1 formation.
Saints created the better early chances - Dave Mackay's low cross zipped in front of Kenny Deuchar but the striker missed his kick before Steven Milne turned and shot just wide from 10 yards after Danny Grainger's long throw.
Motherwell threatened when O'Brien got a toe to Hateley's low cross to the edge of the box, only to see the ball spin just wide with Main scrambling.
Motherwell had a let-off when goalkeeper John Ruddy appeared to claw Stuart McCaffrey's header from inside his goal after Deuchar had headed Morais' deep corner into the danger zone.
Referee Dougie McDonald gave a corner on the advice of his assistant but any sense of injustice disappeared when Saints took a 33rd-minute lead seconds later. Another Morais delivery fell to Davidson 20 yards out and his shot hit a Motherwell player and looped over Ruddy.
Davidson soon had another chance after a clever move involving Liam Craig and Mackay, but Ruddy held his sidefooted effort from 12 yards.
Gannon brought off experienced pair Stephen Craigan and John Sutton at half-time in a bold bid to increase his side's attacking threat. Teenage striker Robert McHugh and winger Chris Humphrey came on as Gannon moved to a 4-4-2.
The switch almost paid off within two minutes when Murphy's low cross beat the home defence but McHugh could not connect and O'Brien sliced wide.
Motherwell were level in the 55th minute thanks to a superb header from Hutchinson. The young defender rose 16 yards out to guide Stevie Hammell's free-kick into the top corner before being booked for celebrating with the visiting fans.
Murphy then broke clear down the right but Main blocked his low drive, and Motherwell's vulnerability at high balls then cost them on the hour mark.
A corner from Morais corner caused chaos and Gartland bundled home from close range after Deuchar had fired into a ruck of bodies.
Motherwell had the ball in the net 90 seconds later but Murphy was booked for handball after meeting Humphrey's powerful cross at the near post.
They were level in the 65th minute when Forbes scored his fourth goal of the season, which for Motherwell began in European competition.
The 20-year-old cut inside from the right wing before curling the ball over Main and into the top corner from 25 yards.
Reynolds produced a brilliant last-ditch challenge after Deuchar was played clear and the former Gretna striker soon missed a glorious opportunity after a catalogue of errors in the Motherwell defence.
Last-man Hutchinson lost possession to Davidson and Ruddy mis-hit another chance to clear before Deuchar volleyed the midfielder's cutback over from 18 yards with the goalkeeper stranded.
Motherwell were also going all out for victory and Craig produced a crucial block from Humphrey before Reynolds headed inches over from a Forbes corner.
Saints substitute Collin Samuel turned Hammell but blasted over from 12 yards and Ruddy saved well from Davidson's powerful long-range strike.
Motherwell went closest to snatching a late winner when O'Brien's first-time volley from 25 yards flew past a motionless Main but bounced to safety off the inside of the post.