Tonio Teklic handed the visitors a 21st-minute lead before Iheanacho equalised with a stunning long-range strike nine minutes from time.
However, Zdravko Dimitrov struck in stoppage time to secure victory for Mardin.
The Nigerian forward has now scored in nine successive league appearances for Bursaspor, registering 12 goals and two assists this season.
He has contributed to 82.4% of his team's 17 goals.
Iheanacho, who joined from Celtic last summer, also scored in Nigeria's recent 3-3 draw against Russia.
The defeat leaves Bursaspor three points behind leaders Kayserispor, with an away fixture against Muglaspor scheduled for Thursday, October 15.
