Armitage, who made his England debut against Italy in the 2009 RBS 6 Nations, scored a try in a man-of-the-match performance while former England fly-half Jonny Wilkinson kicked 17 points in a flawless performance with the boot.
Frenchmen Sebastien Tillous-Borde and Benjamin Lapeyre also scored tries for the hosts at the Stade Felix Mayol but Harlequins could only muster a penalty by fly-half Rory Clegg and a late consolation try from replacement Danny Care.
Harlequins were short of several of the biggest stars, namely England captain Chris Robshaw and New Zealander Nick Evans but did have Nick Easter, Mike Brown and Jordan Turner-Hall in their ranks.
Armitage, 26, has won five caps for England but his decision to leave London Irish for Toulon in the summer 2011 placed him in international exile.
The Rugby Football Union has said that only in 'exceptional circumstances' can players who play abroad turn out for England.
Head coach Stuart Lancaster selected Tom Palmer, who plays his club rugby for Stade Francais, for four of England's five 2012 RBS 6 Nations matches but the lock is returning to England to play for Wasps for the start of next season.