On a cold, blustery afternoon the first shot of the game came in the fifth minute but was a tame effort from Willo Flood that crept past the post.
In the 14th minute, United had a claim for a handball penalty denied but almost took the lead from the resulting corner.
Craig Conway's corner was met full on by Jon Daly whose goalbound header was superbly clawed away by Mike Fraser in the Caley goal.
In the 40th minute United went ahead with a simply-worked and well-taken goal.
Flood made his way down the right flank and slipped a lovely pass out to Sean Dillon, whose first-time cross found Daly alone with time to direct his header into the bottom left-hand corner of the net with Fraser rooted to the spot.
Inverness brought on Barry Wilson for Ian Black at the start of the second period, but it was United who went straight on to the attack and a lovely Conway cross was met by Francisco Jose Sandaza and his powerful downward header was tipped past the post by Fraser.
It was all United at this point and a Paul Dixon cross was met by Darren Dods but his header was high into the sky.
However, when it cam down Lee Wilkie was there to power a header into the roof of the net to double the advantage.
To Thistle's credit they refused to throw in the towel and the Terrors were denied a fourth consecutive clean sheet in the dying moments.
As Caley pressed in the last few minutes they were rewarded with a fluke own goal by Wilkie.
Wilson's corner into the heart of the crowded box hit the United captain on the leg and bounced over the line.