The midfielder, 21, played and scored in Rangers' 2-1 reserve defeat against West Ham on Tuesday.
Mellis, who spent last week training with npower 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.
Source: PA
Source: PA