Higdon completed his hat-trick when Dargo picked him out with a ball from wide on the right.
The striker sealed the points inside a 10-minute period, latching on to a Craig Dargo pass and firing home.
With two minutes left of regulation time, Craig Dargo fired in a shot but Howard managed to parry before smothering the ball.
Inverness will now turn their attention to finding a replacement for Dargo.
BBC: SPORT | Football | My Club | Inverness CT | Dargo turns down Inverness offer
Dargo replaced Mehmet and made an immediate impact as he tried to reach a long ball and was pulled back by Danny Wilson.
Goalkeeper Graeme Smith came off his line to block Dargo's effort as the striker ran in on goal shortly before the deadlock was broken.
Craig Dargo meandered across the edge of the box and set up Billy Mehmet for a drive, which shaved the outside of the left-hand post.
Kevin Thomson was sent off for a dangerous tackle on his namesake Steven and Danny Wilson saw red for a professional foul on Craig Dargo.
Saints hit back and Andy Dorman met Craig Dargo's headed pass following a great break by Stephen McGinn but his 20-yard volley was too high.
Dargo's contract is up and he has hinted recently he may be looking to leave, but Christie is hoping he can keep him at the Highland club.
BBC: SPORT | Football | My Club | Inverness CT | Christie offers Dargo record deal
Craig Dargo volleyed over the bar after Diamond had failed to deal with Jack Ross's searching ball into the area as the Buddies enjoyed a rare period of sustained pressure.
Brady sent a free-kick to the edge of the penalty area, where McGregor headed the ball behind the Hibs defence and Dargo stole in to slot the ball wide of the exposed Brown.
As Dargo tried to catch the pace of the game, he was booked for clipping the heels of Accies captain Alex Neil, then just a minute later slid in on the same player.
BBC: Alex Neil (r) and Steven Thomson in action at St Mirren Park
Lennon swapped Dargo and McAusland for Wardlaw and Kenneth McLean but they had no time to make any impact before Killie scored their first goal, one that will have left Lennon quivering with rage.
Accies substitute Marco Paixao replaced full-back Trent McClenahan as the Lanarkshire side tried another route to victory, while St Mirren introduced Craig Dargo for Higdon, but there was no immediate prospect of the opener at either end.
BBC: Alex Neil (r) and Steven Thomson in action at St Mirren Park
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