In a game of few chances, Marney flicked home a header from David Galbraith's cross just as the referee's assistant held up the board to show two minutes of added time.
It was no less than Colin Calderwood's young outfit - with the exception of Les Ferdinand and Chris Perry - deserved after dominating the first half but going in a goal behind and defending stoutly as the home side pushed on after the break.
In fact, the lads were only a poorly-timed Diego Bortolozzo tackle away from taking all three points.
Gary McSheffrey, City's biggest danger until he limped off at half-time, made no mistake from the penalty spot with only 10 minutes on the clock and it looked like that would be enough until Marney's late, late show.
We started brightly and Lee Barnard had already fired just wide when Ferdinand produced what turned out to be, barring the goals, the moment of the night.
Les, who played 85 minutes, took a pass from Rohan Ricketts, turned away from Craig Pead and lashed a low curler towards the bottom right corner. It was a carbon copy of his goal at Everton on Saturday - this time goalkeeper Gary Montgomery tipped the shot around the post.
The home side then took the lead against the run of play. It was a goal from nowhere as well as McSheffrey pounced on a nothing ball in the box only for Bortolozzo to blight an otherwise tidy night by diving in and giving away a penalty. McSheffrey picked himself up and sent Lars Hirschfeld the wrong way from 12 yards.
Play switched back to other end for the next 20 minutes as Bortolozzo, Ricketts and John Jackson dictated play with Ferdinand, Barnard and the hard-working David Galbraith all linking up.
But with veteran Steve Walsh marshalling City's defence, chances were sparse and Stephen Kelly, Bortolozzo and Barnard were all off target from distance.
Lee Mills had the best chances of the rest of the game either side of the interval. First he shot into the side-netting after capitalising on Kelly losing his footing just before the break and then, with 57 minutes on the clock, he found himself through on goal but couldn't beat Hirschfeld one-on-one.
It wasn't quite happening for us in this final period but we kept going and got our just reward in injury time.
Chris Perry found Galbraith with a searching ball over the top and the wing-back did well to first reach it, then control and force a corner.
He took the corner quickly to George Snee, got the ball back and dinked in the perfect cross for substitute Marney to flick his header low into the corner from eight yards.
Coventry City Reserves: Montgomery, Pead, Quinn, Cooney, Walsh, Jephcott, Pipe, Betts, Mills, McSheffrey (Osbourne, 45), Stanford. Unused subs: Grant, Brush, Giddings, May.
Spurs Reserves: Hirschfeld, S Kelly, Henry, Ricketts (Marney, 64), Bortolozzo (Bowditch, 64), Perry, Snee, Barnard, Ferdinand (Sutton, 85), Jackson, Galbraith. Unused subs: G Kelly, Hughes.
Dean Marney's injury time header rescued a deserved point for our reserves in their FA Premier Reserve League opener at Highfield Road on Tuesday night.