Abbey, the former Cambridge and Norwich striker, has been struggling to recover from a serious knee injury over the last 18 months.
Torquay boss Leroy Rosenior, who has given him a chance after failed trials elsewhere, threw him on with 20 minutes left of what he described as: "A must win game."
Abbey breathed new life into Torquay's tiring attack which had already been robbed of leading scorer Adebayo Akinfenwa with a hamstring injury at half time.
And eight minutes from time it was Abbey who threw himself at a right-wing cross from Darren Garner to head the winner from eight yards.
It was no more that Torquay deserved after a winless run stretching back nine games.
They dominated the second half against a Dons side which had been unbeaten in 11 games.
Only goalkeeper Matt Baker and a series of goal line clearances kept Torquay out.
Baker produced two brilliant saves to deny striker Leon Constantine.
The Dons also never got to grips with man-of-the-match Martin Phillips, who gave a sparkling display on Torquay's right-wing.
Phillips was given a standing ovation when Rosenior took him off in stoppage time.
An indication of the balance of play was that the Dons had only one corner in stoppage time and one shot at goal in the entire second half.
Torquay had taken a risk by playing only one striker up front in Akinfenwa.
Constantine was dropped into midfield for most of the match to use his height to combat the Dons' tall target man, Clive Platt .
The tactic worked, for Platt was never able to oil the wheels of a disappointing Dons' attack.
Torquay made most of the few chances in a dour first half but they moved up a gear after the interval, penning MK Dons back in their own half for the whole of the second period.
And without Baker's heroics it would have been over long before Abbey's winner.