Russell Duncan had the first shot but Chris Smith saved cleanly. Billy Mehmet, Alan Reid and David van Zanten combined to create Saints' first chance but Hugh Murray's header sailed well wide.
Andy Millen and Reid had further unsuccessful attempts but Thistle had the bulk of the pressure, prompted by the lively Zander Sutherland. Rory McAllister and Craig Dargo - twice - had excellent opportunities to score but blazed their shots wide with the goal gaping.
John Sutton nearly made them pay when his sizzling angular drive flew inches wide and Michael Fraser had to be alert to smother another attempt from the same player.
Richard Hart and Richard Hastings had free-kicks saved after the break and Ross Tokely's header struck a post as Caley went for the three points. Smith made a fabulous save, stretching backwards to tip a fierce long-range drive from Hart over his bar.
Ian Black had a half-volley saved by Smith but the keeper was beaten when Stuart McCaffrey beat him to Hart's cross and the centre-back's glancing header floated into the net.
Alex Burke had two shots saved in quick succession as Saints tried to salvage a point and Ian Maxwell was denied by the busy Fraser.
Despite their dismay, the Saints fans gave Kirk Broadfoot, whose superhuman efforts were instrumental in keeping his side in the SPL, a deserved rousing send-off.