John Christensen was drafted by the New York Mets in the second round of the 1981 amateur draft. After spending three seasons in the minors where he batted over .300 he was brought up to the Mets as a September call up in 1984. In his Met debut on September 13th, he pinch hit for Darryl Strawberry with the Mets trailing 9-1 and manager Dave Johnson deciding to wave the white flag. For the record, Christensen walked. He would appear in just five games for the Mets in 1984 batting .273 (3 for 11) with 3 RBIs.
Christensen made the Mets Opening Day roster out of spring training as a fourth outfielder but struggled immensely and was demoted back to the minors in the middle of July after hitting just .190. He did manage to hit 3 home runs and drive in 13, the highlight coming on June 12th when he hit a 2-run home run in the top of the 11th inning to give the Mets a comfortable 7-3 lead. He continued to struggle back at Triple A Norfolk, hitting just .212, but was brought back to Queens in September to finish out the season. In 51 games for the Mets in 1985 Christensen hit just .186.
He was traded by the Mets in the off-season along with Calvin Schiraldi, Wes Gardner and LaSchelle Tarver to the Boston Red Sox in exchange for Bob Ojeda, John Mitchell, Tom McCarthy and Chris Bayer.
His final numbers with the Mets included a .194 average with 3 home runs and 16 RBIs in 56 games.
While with the Mets Christensen wore number 35 in 1984 and number 7 in 1985.
John Chrsitensen also played for the Seattle Mariners in 1987 and the Minnesota Twins in 1988.