Mickey stayed behind in a parallel universe after the events of "The Age Of Steel" but returned to our universe in "Army of Ghosts"/"Doomsday" when the Cybermen broke threw to our universe and started a war with the Daleks. He then got trapped in the parallel universe again with Rose when the Doctor rebuilt the walls of the universe. Mickey returned in "Journey's End" when the walls of universe were yet again falling apart as a result of Davros' Reality Bomb. After helping the save the universe, he decided to stay in our universe and left with Martha and Captain Jack. He was last seen in "The End Of Time", Part 2 when the Doctor was saying goodbye to all his friends. He had married Martha Jones and become a freelance alien fighter. He was fighting a Sontaran with Martha when the Doctor saved their lives. He hasn't been seen since.