The midfielder, 21, played and scored in Rangers' 2-1 reserve-team defeat against West Ham on Tuesday.
Mellis, who spent last week training with Championship side Burnley, left Chelsea "by mutual consent" last month after admitting to the smoke-grenade prank.
Tuesday's friendly also witnessed the return of QPR strikers Heidar Helguson and DJ Campbell from injuries that have sidelined them for more than two months.
Former Sheffield United trainee Mellis, 21, was dismissed after a "full and thorough investigation" of an incident on March 2.
It is understood that Mellis set off the grenade, which was brought into the dressing room by Billy Clifford, 19, after a paintball session.
At the time of the incident terrified players and staff members ran from the building, with fire marshals later brought in.