Mitchell led the squadron of F-302 fighters in the battle against Anubis’s forces in Antarctica (“Lost City, Part 2”). He was shot down and badly injured, but promised a place on SG-1 should he recover (story). Mitchell will likely be assigned as SG-1’s new commanding officer in the absence of Lieutenant Colonel Carter (played by Amanda Tapping, who will not appear in the first five episodes of the new season due to the birth of her child).
It is not yet clear which of the two characters, who hold the same rank, will lead the team when Carter returns.
TV Guide previously reported that the initials of Browder’s new character were “M.M.,” and that the producers couldn’t reveal it until it was cleared through legal channels. The character’s first name was probably changed during that process so as not to resemble an actual person.