Is there any way to cancel my mind control on an enemy alien?
How can I kill an enemy under mind control?
I have the final alien in a mission mind controlled and the mission isn't completing. Is there any way to cancel the MC rather than waiting around for it to blast one of my team members?
Nope, there's no way to cancel it. You've just got to wait for the mind control to wear off, or for the alien to die.
Of course, you can speed up their death through the use of free-aimed explosives (rockets and grenades and such), but otherwise there's not much you can do.