Wycombe took the lead through Brandon Hanlan, but Kevin de Bruyne levelled seven minutes later.
Mahrez claimed his double either side of half-time, with Phil Foden and Ferran Torres also beating veteran goalkeeper David Stockdale.
Substitute Cole Palmer, 19, added the sixth with his first goal for the club.
It was a massive night for City’s youngsters, with Pep Guardiola handing senior debuts,
to academy graduates Conrad Egan-Riley, Luke Mbete, Finley Burns, Josh Wilson-Esbrand and Romeo Lavia.
Defender Wilson-Esbrand was particularly impressive, with his excellent control and cut-back assisting Mahrez for City’s second strike.