Christenson would bounce back and forth from the majors to the minors until 1975, when the Phillies called him up to stay. He went 11–6 that season and would become a key cog on Phillies teams that would win three straight NL Eastern Division titles (1976–1978). Christenson would have his best seasons those three years: 1976, going 13–8 with a 3.68 earned run average (ERA); 1977 (his best season), when he went 19–6 with a 4.06 ERA, winning 15 of his last 16 decisions; and 1978, where he slipped to 13–14, despite posting a career-best ERA of 3.24. In the 1978 National League Championship Series, Christenson was the Phillies’ Game 1 starter.