Mahrez spent five years at Manchester City and helped the club win 11 trophies, including four Premier League titles and the UEFA Champions League.
City agreed a fee with Al-Ahli last week that is worth up to £30m for the Algeria international.
Mahrez, 32, spent five years at the Etihad Stadium and helped the club win 11 trophies, including four Premier League titles and the Champions League.
Mahrez told mancity.com of the title winners: “It was an honor and a privilege to play for Manchester City.
“I came to City to win trophies and enjoy my football and I achieved all that and more.
“I have had five unforgettable years at this football club, working with incredible players, fantastic fans and the best manager in the world.
“Manchester City will forever be a huge and happy part of my life.”