Near the end of the first half at Ankara's Eryaman Stadium, Ankaragucu were awarded a penalty shot when Galatasaray midfielder Arda Turan was whistled for a handball in the penalty area.
Ankaragucu midfielder Ibrahim Akdag successfully converted the penalty kick and his team went into the break with the 1-0 lead.
The home team doubled the gap in minute 55 as Georgian winger Saba Lobjanidze skipped Brazilian defender Marcao and chipped the ball above Galatasaray goalkeeper Fernando Muslera.
Two minutes later, Galatasaray were down to 10 men when Egyptian forward Mostafa Mohamed was sent off for elbowing Akdag.
The red card placed Galatasaray in a precarious situation with the league leaders facing a loss to a team that is on the cusp of relegation.
The visitors managed to close the gap in injury time when Turkish left winger Muhammed Kerem Akturkoglu converted on a one-on-one chance in the penalty area.
The loss means that Galatasaray remain at 57 points in 27 matches atop the league.
Ankaragucu increased their points to 23 and are in the nineteenth spot in the standings.
Galatasaray's winning streak ends
Ankaragucu's win ended Galatasaray's eight-match win streak in the Super Lig.
The Lions last lost 2-0 to Besiktas on Jan. 17.
After the derby defeat against Besiktas, Galatasaray had a run of wins against Yukatel Denizlispor, Yeni Malatyaspor, Gaziantep FK, Medipol Basaksehir, Fenerbahce, Kasimpasa, Aytemiz Alanyaspor and Buyuksehir Belediye Erzurumspor.
Ankaragucu's first win in 6 games
The victory was Ankaragucu's first victory during a stretch where the team went on a six-game winless run.
The Ankara club previously lost to Aytemiz Alanyaspor, Buyuksehir Belediye Erzurumspor, Demir Grup Sivasspor and Atakas Hatayspor.
They drew against Hes Kablo Kayserispor and Caykur Rizespor.
Ankaragucu earned their last win in a match against Kasimpasa by 1-0 on Jan. 21.
Wednesday's results:
Buyuksehir Belediye Erzurumspor - Fatih Karagumruk: 2-2
Demir Grup Sivasspor - Atakas Hatayspor: 1-1
Medipol Basaksehir - İttifak Holding Konyaspor: 1-1
Hes Kablo Kayserispor - Caykur Rizespor: 2-1
MKE Ankaragucu - Galatasaray: 2-1
