Ayr United slipped into a relegation play-off spot after a third defeat in a row, as they finished with 10 men.
Partick Thistle, who twice struck four past their opponents this season, went ahead when Chris Erskine converted Paul Cairney's cross in the 17th minute.
The visitors doubled their lead 12 minutes before the break courtesy of Kris Doolan.
Erskine struck a post with a shot from the edge of the area but Cairney was on hand to centre the ball and Doolan produced an emphatic finish from close range.
The home supporters were given hope 11 minutes from time when substitute Adam Dodd lashed a left-foot shot into the top corner.
But Dodd turned from hero to villain after being dismissed for violent conduct and Mark McGuigan wrapped up the points with a third Thistle goal, following up after Aaron Sinclair's initial shot was saved.
Cuthbert, Tiffoney, Malone, John Robertson, Smith, Geggan, McGowan, Tomsett, Moffat (Parker 54), Roberts (Wardlaw 69), Trouten (Dodd 54). Subs Not Used: McWilliams, Martyn Campbell.
Fox, O'Donnell, Sinclair, Paton, Robertson, Archibald, Cairney, Bannigan, Doolan, Welsh, Erskine (McGuigan 73). Subs Not Used: Scully, Elliot, Balatoni, McAleer.
Erskine 27, Doolan 33, McGuigan 90.