Information about Mitchell's past is revealed over the course of several episodes in Seasons 9 and 10 of Stargate SG-1. Collateral Damage flashbacks to Mitchell's father, a test pilot for the Air Force whom Mitchell would admit years later for his coping with losing both legs in an accident. The episode Bounty reveals that Mitchell graduated in 1987 from a high school in Auburn, Kansas, which is where his parents currently reside. During high school, Mitchell had a crush on one Amy Vanderburg, which he only finds out at their 20-year reunion that the crush had been mutual. Mitchell sometimes mentions his Bible-thumping grandmother.

He also have dealt with a wrong bombing incident when he was a F-16 pilot with rank of Captain. Almost immediately after Mitchell fired, the orders were withdrawn because the convoy was actually refugees fleeing to safety. With the bomb finding its mark, Mitchell was still wracked with guilt and almost resigned. The premiere of season 9, Avalon, flashbacks to Lt. Colonel Mitchell leading the squadron of F-302 fighters in the battle against Anubis' forces in Antarctica at the end of Season 7, where he was shot down. Mitchell received the Medal of Honor for his actions over Antarctica, severe injuries risked his ability to ever walk again, but he was promised any assignment he wanted should he recover, and chose to take a place on SG-1.

Upon making his entry at the SGC in Avalon, Mitchell is assigned as SG-1's new commanding officer. Learning that the rest of SG-1 has moved on, Mitchell makes several valiant attempts to lure them back to SG-1 but remains unsuccessful at first. Mitchell leads former SG-1 team members Daniel Jackson and Teal'c, and a human alien named Vala Mal Doran on a quest for knowledge (and riches), but it is not until the threat of the Ori becomes a real threat to Earth that former SG-1 member Samantha Carter joins them.

SG-1 reunites under his command in the seventh episode of Season 9, Ex Deus Machina. After Mitchell becomes the first human to make contact with the legendary Sodan, a group of Jaffa, in Babylon, he is also present when the Ori kill the race in Arthur's Mantle. In Collateral Damage, Mitchell is implanted with a memory of bludgeoning a woman to death and must defend himself against accusations, but his innocence is later proven.

Source | Wikipedia.org