The Buddies looked set to collect their first three points of the season after Kenny McLean had given them a early second-half lead.
The League Cup holders were already reeling this week from an early exit to Queen of the South in the competition they won just six months ago.
The Saints boss had hoped his new loan signing from Nice, Stephane Bahoken, would add some much-needed extra firepower to his side.
But after throwing him straight into his line-up despite him having landed in Scotland just 24 hours before the match, Lennon was forced to hook him off at half-time after an ineffectual display.
Yet Bahoken had looked desperate to make an instant impact in the early stages.
His desire was evident after just five minutes as he latched on to Steven Thompson's flick-on from keeper David Cornell's long punt and immediately looked for the target.
His next strike after 23 minutes, however, was a wild attempt which flew both high and wide of the Thistle goal but he did at least force Fox to stoop and gather a couple of minutes later after fizzing in a low drive from the edge of the box.
However, blood was still rushing to the 21-year-old's head when he tried again after spinning past Aaron Muirhead before taking a reckless swing at the ball which sent it nearer to the corner flag than the Jags stopper.
Partick claimed for a penalty 15 minutes before the break when home skipper Jim Goodwin barged over John Baird in the box but referee Willie Collum correctly saw the coming together as shoulder to shoulder.
Bahoken then dragged another shot wide of target as his frustrations began to show, while Thistle were disheartened when James Craigen wasted their best opening of the half by heading just over after Steven Lawless and Stuart Bannigan combined for the latter to swing in a perfect cross to the back post.
The visitors came even closer a minute before half-time as right-back Stephen O'Donnell lofted in a high ball for his opposite full-back Aaron Taylor-Sinclair to nod down towards Lawless, but the little midfielder zipped a volley an inch beyond Cornell's left-hand post.
Bahoken's increasingly wayward display convinced Lennon that enough was enough and he was replaced just 45 minutes into his Paisley bow, with Gary Teale replacing him.
Yet Thistle continued to threaten as Baird pounced on a McLean slip before firing wide just moments after the restart.
But Teale's introduction soon proved crucial as he collected David van Zanten's throw-in before rolling it down the line for the overlapping Irishman to cut back a low cross which McLean took first-time on the half-volley, sending it under Fox's diving reach for the opener.
The tempo had dropped from the frantic pace of the first 45 but the chances continued to fall for both sides. First Cornell had to punch clear from Craigen's fierce volley while at the other end, McLean shot over from Teale's quick free-kick.
Cornell was alert again to deny Partick substitute Kris Doolan from the edge of the box but the Welsh keeper had no chance as Higginbotham curled home the leveller.
The Englishman played a swift one-two with O'Donnell on the right before stepping in towards the angle of the box and then shaping a well-struck finish past the St Mirren number one at the far post.
And there was still time for a winner for the Maryhill men as Doolan controlled a high ball into the box before rolling it back to fellow replacement Forbes, who drilled home the finish from 15 yards.
- Sports & Recreation
- David Cornell
- Kallum Higginbotham