After pitching with the Athletics in spring training, manager Connie Mack placed Christopher on the major league roster, and he made his debut on April 14, 1942. On May 5, Christopher made his first appearance as a starting pitcher against the Detroit Tigers. The Athletics won the game, 2–1, and he got his first major league win. After another win five days later, he began a long losing streak. Many of the games he did pitch were in relief, including a game in June against the Cleveland Indians where he only allowed one hit in three innings. He got his third win of the season three months after his second on August 20 in a 2–1 victory over the Washington Nationals. He finished the season with a 4–13 record, a 3.82 ERA, 30 total games, 58 strikeouts and 99 walks. The following season, Christopher re-signed with the Athletics, and was one of the few remaining pitchers from the previous year on the team, the rest having been called to serve in World War II. He began the season in the starting rotation, winning his first start of the season against the Boston Red Sox in a 5–0 shutout. Over the course of the season, he converted to a sidearm pitching delivery, which led to three victories in a row for him. After a July 9 loss to the Tigers brought his record to 4–6, Christopher did not pitch again until August 24, and spent the rest of the season pitching in relief. He finished the season with a 5–8 record and a 3.45 ERA.