The visitors were first to settle, and after nine minutes Martin Cameron took advantage of some slack defending by Sandy Hodge, but his cross failed to find any team mate.
Forfar continued to play the better football, but they took a further 14 minutes for their next attempt at goal when David McClune's 18-yard shot failed to trouble Kenny Arthur.
Thistle were struggling to produce any attacks. However, they should have taken the lead after half an hour when a Jimmy Gibson cross was met at the back post by Billy Dodds whose header fell right at the feet of Paul Ritchie, who somehow managed to shoot over from two yards.
This opportunity seemed to spur Thistle, and four minutes later Darren Brady had their first effort on target but the shot was easily saved.
The second half saw Thistle come out fired up, and after 47 minutes a 20-yard shot from Billy Gibson was parried for a corner. From the resultant flag kick, Dodds looked on in despair as his header hit the wrong side of the far post and was cleared.
Thistle were enjoying the lion's share of possession, and they made their first substitution on 61 minutes. This proved to be an inspired move when, four minutes later, substitute Adam Strachan fired them into the lead.
Forfar were unable to clear a Scott McCulloch free-kick and the ball fell kindly to Strachan 16 yards out, and he fired an unstoppable shot low into the far corner.
The same player almost doubled his tally after 69 minutes, but he shot well over after being played in by Dodds.
Despite continued efforts Forfar were unable to equalise, and Thistle ran down the clock for a victory that maintains their play-off chances.