He had his first taste of professional baseball in 1938, when he signed with the Clovis Pioneers of the West Texas–New Mexico League. He pitched in 13 games for the Pioneers, posting a 7–5 record, 106 innings pitched and a 4.50 ERA. The following year, Christopher joined the El Paso Texans, the rookie-level minor league team of the New York Yankees farm system. In 32 games with the Texans, he posted an 18–7 record, pitched 225 innings and had an ERA of 3.68. He was promoted in 1940 to the Wenatchee Chiefs of the Western International League, where he finished the year with an 8–8 record and a 4.72 ERA in 20 games. At the end of the season, he was promoted to the Newark Bears of the International League along with seven other minor league players.