The hit-man's clinical finish made the difference in a poor game which was spoiled by the windy conditions.
It was, of course, the first time United manager Craig Brewster had taken his side north since he left the Highland hot-seat in January.
And he had to do without Lee Miller, David Fernandez, Derek Stillie, Jim McIntyre and the suspended Barry Robson.
Dennis Wyness returned to the home line-up after a hamstring injury.
The first chance of the game fell to impressive United left-sider Charlie Mulgrew, who swerved a fine free-kick just around the post. The ace, on loan from Celtic, was in fine form and seemed the man most capable of magic for United.
Then on 19 minutes a great diving save from goalkeeper Craig Samson denied Caley Thistle midfielder Richie Hart, whose side-footed drive was pushed wide for a corner.
After a long period of dull moments, Dargo then almost connected with a headed flick-on from Darren Dods.
No goals in the opening 45 minutes left chilled fans in the windy breeze hoping for much more entertainment after the interval.
Mulgrew was twice in the right place in the early stages of the second half to deny Dargo and then Barry Wilson.
Down the other end, Alan Archibald and Paul Ritchie threatened the Inverness area, without penetrating it.
Late on, Inverness sub Alan Morgan flashed a shot wide after smart work from Ross Tokely on the right flank.
But, the 0-0 scoreline looked on the cards until Dargo netted with the last kick of the game after he got the better of Alan Archibald at the back before smashing the ball into the net.