Captain Mitchell was a Merchant Marine and the captain of the ocean freighter Achilles, which was responsible for transporting Earth's primary Stargate to the US in 1939. While Mitchell and his crew did not know how important his cargo was, all he understood was that it was considered valuable by the U.S. Government and had to be extracted from Egypt before the Nazis found it. One night, while his ship was nearing the South Pole, Mitchell and his crew heard noises coming from the ship's hold (really the Stargate's chevrons activating, courtesy of an imminent attack by the deposed Goa'uld System Lord Ba'al using a time-travelling device in an effort to stop the normal flow of history by ensuring the Stargate wasn't delivered, which by extension would mean the Stargate Program would never exist. Mitchell sent a crewman down to check it out, who then teamed up with another crewmember; an alternate version of Mitchell's grandson; Cameron, who had been preparing for 10 years to stop Ba'al from changing the timeline. Utilizing Thompson machine guns, Cameron and the crewman shot Ba'al and his three Jaffa warriors dead upon their arrival, saving the timeline. Mitchell himself arrived at the end of the battle, and agreed to cover up what had transpired to preserve history. Mitchell has since died of natural causes, but a photo is kept by the mainstream version of Cameron, unaware that in the photo his older alternate self stands at Mitchell's side, having become friends during his time in the past.
Alternate Timeline
In an alternate timeline, Ba'al and his three Jaffa warriors came aboard unapposed, and set about securing the ship for destruction, proceeding to massacre the crew. While Mitchell was shot with a staff blast, he survived his injuries and, upon getting his gun, headed towards the cargo hold to confront the attackers, though his injuries slowed him down and Ba'al and his Jaffa escaped, leaving behind a powerful Goa'uld explosive orb device. Though he did not recognize it, Mitchell knew enough that it was some kind of explosive, and promptly tossed it through the hole made in the hull by the Stargate's activation and overboard, with mere seconds to spare. Mitchell then died of his injuries, the last of his family line as by then he hadn't fathered any children. Meanwhile, the Achilles drifted aimlessly and was lost in the South Pole, it's Stargate lost and the Stargate Program never existing.