Teenager Mark Stewart had given John Hughes' injury-hit side the lead on the hour-mark against the run of play, before Don Cowie drove home a loose ball from a corner eight minutes later.
It seemed a draw was on the cards, but Imrie, who been on the field less than three minutes, took advantage of more defensive uncertainty at a corner to grab the three points for the Highlanders.
It was probably what Craig Brewster's team merited after a combination of wasteful finishing and the saves of Robert Olejnik had thwarted them, especially in the first half.
But for Hughes it was back to the drawing board as his charges remained rooted to the foot of the table.
Falkirk were dealt a blow before kick-off when top scorer Michael Higdon was ruled out, although Graham Barrett at least returned for his first start of the season.
Summer signings Steve Lovell and Burton O'Brien were also fit enough to make the bench as Hughes' lengthy injury list began to clear.
Inverness, meanwhile, recalled Michael Fraser, Roy McBain, Adam Rooney and Richard Hastings from the team that narrowly defeated Arbroath on penalties in midweek.
Hughes had served the final game of his six-match touchline ban at Raith on Tuesday, yet he chose to again sit himself high in the stand for the opening half.
Referee Mike McCurry, taking charge of his first top-flight match of the campaign, must have hoped for a stress-free afternoon after Hughes' high-profile public spat with officials.
But the Bairns boss was surely far from impressed at the early proceedings, with his defence, rather than the referee, the source of his ire.
Lee Bullen was forced to hack clear from a dangerous Garry Wood cutback following an earlier defensive slip, before Olejnik came to Falkirk's rescue with a superb double save after seven minutes.
First he denied Inverness new boy Rooney, who had strode unopposed through the middle, before the Austrian quickly flung himself to his right to push away Woods' follow-up.
Mark Stewart replied with a surging run and shot but the effort was too weak to trouble Fraser.
John Stewart came closer in the 20th minute, heading just past after a neat centre from Scott Arfield.
But Falkirk continued to look shaky in defence and captain Darren Barr was almost punished before the half hour.
Wood's ball forward appeared to have been dealt with by the 23-year-old, but he failed to control and was robbed by Rooney, who bore down on Olejnik's goal.
While the keeper made another fine stop, the recent acquisition from Stoke should have done better.
Falkirk were again lucky to escape after 37 minutes. As defenders stood off, Cowie was allowed to clip a shot towards the top corner that Olejnik appeared to get a touch on, despite a corner not being given.
The Bairns, lightweight in attack without Higdon, remained second best, but Barrett's curling cross was crying out for a team-mate's head two minutes from the interval.
Hughes returned to patrol his dugout at the restart, but still Inverness remained the more dominant force and Rooney soon nodded over a Cowie corner.
In the 58th minute, Ian Black rose unopposed to head an Iain Vigurs corner goalward but it was easy for Olejnik.
Falkirk finally replied with an Arfield 20-yarder that Fraser grasped at the second attempt, before they made the breakthrough against the run of play.
Barrett brilliantly chased a loose ball down and his cutback from the byline allowed Mark Stewart to expertly sidefoot into the bottom left-hand corner.
However, it took just eight minutes for Brewster's side to deservedly level.
From Vigurs' corner, Jamie Duff made a fresh air swipe at goal, but with the ball still bobbling around, Cowie struck a left-foot drive through a ruck of bodies and under Olejnik.
With the visitors' tails up, Bullen was forced to nod off his line after Phil McGuire's header back across goal.
Hughes handed Lovell and O'Brien competitive debuts late on in a bid to rouse his team, yet McGuire was left unmarked to direct a header at Olejnik with eight minutes left.
Caley sensed a winner and it arrived five minutes from time, with the home defence again culpable.
From an initial corner, Cowie saw a shot blocked by team-mate Rooney, but Falkirk simply failed to clear their lines and Imrie gratefully lashed a drive into the top left-hand corner.