Mahrez, one of only a handful of senior players available to Pep Guardiola following the European Championships and the Copa America, opened the scoring in the first half against the Championship side before 18-year-old Sam Edozie added a second after the break.
Fernandinho and Mahrez also started at City's academy stadium alongside a host of youngsters including exciting winger Morgan Rogers. Mahrez gave City the lead in the 25th minute with a curling free-kick from 25 yards which was too good for Preston goalkeeper Connor Ripley.
Edozie, on as a substitute at half-time as Guardiola made 10 changes, doubled City's advantage after a slick move after 64 minutes. An otherwise comfortable evening for City was soured late on when 18-year-old goalkeeper Cieran Slicker was forced off on a stretcher after a collision with Tom Bayliss.