The former All Star forward came off the bench in the 40th minute and scored two points to mark his first game in the county colours for 18 months.
The sides were deadlocked at 0-4 apiece at half-time but Mayo produced a strong final quarter with points from Darren Coen, Enda Varley (two frees) and Mortimer to secure victory.
The winners also had Donal Vaughan sent off in the closing stages.
Scorers for Mayo: E Varley 0-4 (3fs), C Mortimer (0-1f), D Coen 0-2 each, C Boyle, D Kirby 0-1.
Scorers for Leitrim: J Glancy 0-2fs, C McCrann (45), P McGowan, D Sweeney (f), G Hickey 0-1 each.
MAYO: D Clarke; S McHale, E O’Reilly, L Keegan; P Gardiner, D Vaughan, C Boyle; B Moran, S O’Shea; K McLoughlin, P Harte, D Kirby; E Varley, D Coen, M Conroy.
Subs: C Mortimer for Harte, G Cafferkey for McHale, R Feeney for McLoughlin.
LEITRIM: C McCrann; A Wynne, C Egan, P McGuire; D Beck, J Glancy, W McKeon; P McGowan, D Lowe; P Brennan, R Lowe, G Hickey; C Clarke, J Glancy, R Cox.
Subs: D Gilhooley for Glancy, D Sweeney for McGowan.
Referee: Michael Duffy (Sligo).