Christenson made his major-league debut on April 13, 1973, and beat the New York Mets 7-1, pitching a complete game. At the time, he was the youngest player in the Major Leagues at 19; he would remain so until 18-year-old David Clyde debuted for the Texas Rangers on June 27 of that same season. He would bounce back and forth from the majors to the minor leagues until 1975, when the Phillies called him up to stay. Christenson went 11-6 that season and would become a key cog on the Phillies' teams that would win three straight National League Eastern Division titles from 1976 to 1978. He would have his best seasons those three years, going 13-8 with a 3.68 earned run average in 1976. His best season was 1977, when he went 19-6 with a 4.06 ERA, winning 15 of his last 16 decisions. He slipped to 13-14 in 1978, despite posting a career-best ERA of 3.24, and started Game 1 of the 1978 National League Championship Series.
Then, injuries would begin to plague Christenson's career. He began the 1979 season on the disabled list with elbow problems and missed the first month. Then, in June of that season, he broke his collarbone during a charity bicycle ride and missed several weeks. He ended up with a 5-10 record that season. In 1980, he started 3-0, but went on the disabled list again and had elbow surgery. He recovered to finish the season 5-1 and start Game 4 of the 1980 World Series, but was knocked in the first inning.
In 1983, Christenson went under the knife for elbow surgery for the final time after a 2-4 start. He failed to make the postseason roster and the Phillies gave him his unconditional release on November 10 of that year, his 30th birthday.
Christensen tried for several years, spent in his home state of Washington, to rehabilitate from his numerous surgeries, but he was unable to return to baseball. He began a career in institutional investing. He is president of Christenson Investment Partners and resides in the Philadelphia area. He has two daughters; Claire, aged 17, and Libby, aged 19. He maintains his ties with the Phillies and is well known locally for his work on behalf of numerous charities.