Christenson made his MLB debut on April 13, 1973, beating the National League (NL)-rival New York Mets, 7–1, while pitching a complete game.

[2] At the time, he was the youngest player in MLB at age 19; he would remain so until 18-year-old David Clyde debuted for the Texas Rangers, that June 27.

In 1981, Christenson posted a less-than-stellar 4–7 record, but notched a win in the 1981 National League Division Series, against the Montreal Expos.

He failed to make the postseason roster and the Phillies gave him his unconditional release on November 10 of that year, his 30th birthday.

Although only a .150 hitter (64-for-427) in his 11-year major league career, Christenson hit 11 home runs with 46 RBI and 24 bases on balls.

Christenson tried for several years, spent in his home state of Washington, to rehabilitate from his numerous surgeries, but was unable to return to baseball.