Ten-man Partick Thistle lost their grip on top spot after a defeat away to Morton.
It was another impressive show from the Cappielow side as they ended Partick's unbeaten record and stretched their own winning run to five games.
Kevin Rutkiewicz gave the home side the lead in the 14th minute as he prodded home Michael Tidser's corner from close range.
The lead lasted just six minutes as Steven Craig volleyed in an equaliser from 12 yards out.
The Ton re-took the lead six minutes before the break from another corner as Mark McLaughlin scored with a header.
The Jags were reduced to 10 men in the 55th minute as Hugh Murray was shown a second yellow for a foul on Stephen Stirling.
And the Greenock men made full use of their advantage as David O'Brien drilled in a low effort from the edge of the area in the 86th minute.
Gaston, Reid, Rutkiewicz, Mark McLaughlin (Naismith 79), Dyer, Stirling (Graham 58), Bachirou, Tidser, O'Brien, Weatherson (Hardie 73), Campbell.Subs Not Used: Hutton, Wallace.
Rutkiewicz 14, Mark McLaughlin 39, O'Brien 86,
Fox, O'Donnell, Muirhead, Archibald, Bannigan, Paton, Murray, Forbes (Slane 69), Lawless (Doolan 69), Craig, Erskine (Sinclair 60).Subs Not Used:Scully, Elliot.
Murray, Paton, Bannigan.