He made his debut out of the bullpen the Dodgers. In 1981, he earned a spot in the Dodgers rotation which would set off in Los Angeles in what is remembered as Fernandomania. He won his first eight starts in the major leagues, and a remarkable five of those were shutouts. In the strike shortened season of 1981, he would make 25 starts, 8 shutouts, and 11 complete games, and finished a won-loss record of 13-7. He was the first to win the Cy Young award, and Rookie of the Year award in same season. The Dodgers won the World Series,
He was a six-time National League all-star, in 1986, he would win the Gold Glove. A good hitting pitcher, Valenzuela would earn Silver Slugger awards in 1981 and 1983. On June 29, 1990, Valenzuela would toss his only no-hitter in beating the St. Louis Cardinals. 1986 was the only season he was a twenty game winner, winning 21 that season.
The Dodgers would release him after a power showing in spring training 1991. After a brief appearance with the California Angels, and spent time in the Mexican league before returning to the major leagues with the Baltimore Orioles. He would never enjoy the same success as he did in Los Angeles. He would finish career 173-153 with an ERA of 3.54. He was inducted into the Mexican Baseball Hall of Fame in 2014.