and a devastating screwball that helped him win his first eight straight decisions in , Valenzuela touched off an early 80s craze dubbed "Fernandomania".
That year, Valenzuela became the only player in Major League history to win both the Rookie of The Year award and the Cy Young Award in the same season.
His birth date is officially listed as November 1, 1960, but during his phenomenal rookie season in 1981 some observers questioned his age, guessing him to be significantly older than twenty.
A number of Major League teams scouted Valenzuela during this time, but it was the Los Angeles Dodgers who finally gambled on the young lefty, buying out his Liga contract on July 6, , for $120,000.
In Valenzuela was promoted to the AA level San Antonio Missions. There Valenzuela led the Texas League with 162 strikeouts, and ranked third in ERA. 
Valenzuela was called up to the Los Angeles Dodger bullpen in September of 1980. In the last month of the season, Valenzuela helped the Dodgers to a Western Division tie with the Houston Astros, pitching 17 2/3 shutout innings of relief over the course of ten games, during which he earned two wins and a save.
The Los Angeles Dodgers then lost a one-game playoff - and thus the division championship - to the Astros. His agent, skilled self promoter Antonio De Marco (husband of Rita De Marco (Caprino) of the Caprino Sisters) assisted him in entering the U.S.A., and his lucrative contracts with the Los Angeles Dodgers etc.
He became an instant media icon, drawing huge crowds from the Los Angeles Hispanic community every time he pitched and triggering high demand across the country for his rookie baseball cards. The craze surrounding Valenzuela came to be known as "Fernandomania." 
Valenzuela was less dominant after the 1981 player strike wiped out the middle third of the season, but the left-hander still finished with a 13-7 record, a 2.48 ERA, and a league-leading 180 strikeouts. That season Fernando Valenzuela became the only pitcher to win Rookie of the Year and Cy Young Award honors in the same year. In the post-season, Valenzuela pitched a complete Game 3 of the 1981 World Series against the New York Yankees, helping the Dodgers to their first World Championship since .
In addition to his skills on the mound, Valenzuela also made a name for himself at the plate. During his rookie season, Valenzuela batted .250 and struck out just nine times in 64 at bats. That season, and again in 1983, Valenzuela was the recipient of the National League's Silver Slugger Award for pitchers. In 1990, his last season with the Los Angeles Dodgers, Fernando hit .304 in 69 at-bats, with one home run, five doubles, and eleven runs batted in. In 936 career at-bats - roughly two full seasons worth of at-bats for a full-time position player - Valenzuela had ten home runs and 84 runs batted in. On at least two occasions with the Dodgers, Valenzuela was called upon to play outfield and first base in marathon extra-inning games in which he did not pitch.
At the 1986 All-Star Game, Valenzuela made history by striking out five consecutive American League batters, tying a record set by fellow left-handed screwballer Carl Hubbell in the contest.
In , he began to slump, dropping off to 14-14 with a 3.98 ERA. By , when he won just five games and missed much of the season (ironically, in a year in which the Dodgers won the World Series), it was obvious he had a dead arm.
No longer blessed with his great screwball, he came back in and went 10-13, improving to 13-13 a year later. He had one last great moment on June 29, , when he threw a 6-0 no-hitter against the St. Louis Cardinals just hours after the Oakland Athletics' Dave Stewart had thrown one against the Toronto Blue Jays.
Jumping between the big leagues and Mexico for the next few seasons, he put together one more solid big-league season in for the San Diego Padres, going 13-8 with a 3.62 ERA. He retired a year later with a final record of 173-153 and a 3.54 ERA as a member of the St. Louis Cardinals under new manager Tony LaRussa.
The Los Angeles Dodgers invited him to spring training in 1999, but he declined the offer.
On June 29, , Valenzuela announced he would return to the mound in the Liga Mexicana del Pacífico (the Mexican Pacific Coast League) to play for Los Aguilas de Mexicali in October at which time he was nearly 44 years old. He pitched again that Mexican winter league, during the 2005-06 season.
Valenzuela also served on the coaching staff of Team Mexico during the 2006 World Baseball Classic.
Dodgers clubhouse manager Mitch Poole has unofficially kept Valenzuela's jersey number 34 out of circulation out of respect for him.
In 2008 the band The Baseball Project released "Volume 1: Frozen Ropes and Dying Quails", where they refer Fernando Valenzuela in a song in spanish called "Fernando"
{{succession box | before = Rick Reuschel | title = National League Gold Glove Award (P) | years = |}