Eight jets took off from the carrier in the eastern Mediterranean, an AFP photographer on the flight deck reported.
The Charles de Gaulle is on its third mission since February 2015 in support of the US-led coalition fighting IS in Iraq and Syria.
It was not immediately clear whether the French jets were to carry out air strikes or were on a reconnaissance mission.
Isis seized Mosul along with other areas in June 2014, but Iraqi forces have since regained significant ground from the jihadists and are readying for an assault to retake Iraq's second-largest city with coalition air and artillery support.
French President Francois Hollande announced in July that the Charles de Gaulle, which carries 24 Rafale jets, would be sent back to the region for bombing raids on Isis targets.
The vessel's latest mission is planned to end in late October, according to Defence Minister Jean-Yves Le Drian.