Reggie Jackson was born in Wyncote, Pennsylvania, just north of Philadelphia, but later made his home in Oakland, California. He graduated from Cheltenham High School in 1964 where he starred in football and baseball and was a classmate of Jonathan Netanyahu. He was a very dedicated scholar. Jackson attended Arizona State University on a football scholarship. He switched to baseball following his freshman year, impressing coach Bobby Winkles with his strength.

He was the second selection in baseball's 1966 amateur draft, chosen by the Kansas City Athletics. The New York Mets had the first pick, but chose Steve Chilcott, a catcher who had just graduated from high school in California. Jackson-- according to his 1984 autobiography-- was told by Winkles that the Mets were nervous about drafting him because he had a white girlfriend; Jackson's girlfriend, Jennie Campos, was actually a Mexican-American, and Jackson himself was of partial Latino heritage, part Puerto Rican and Panamanian. Jackson and Campos later married and divorced, and through 2005, he has never remarried. Chilcott played six years in the minor leagues before retiring.

Jackson debuted in the major leagues with the A's on June 9, 1967, a 6-0 A's victory over the Cleveland Indians in Cleveland. Following that season, the Athletics moved to Oakland. Jackson hit 47 home runs in 1969, and was briefly ahead of the pace that Roger Maris set when he broke the single-season record for home runs with 61 in 1961, and that of Babe Ruth when he set the previous record of 60 in 1927. Jackson later said that the sportswriters were claiming he was "dating a lady named 'Ruth Maris.'" That off-season, Jackson sought an increase in salary, and A's owner Charlie Finley threatened to send Jackson to the minors. Commissioner Bowie Kuhn successfully intervened in their dispute, but Jackson's numbers in 1970 dropped sharply, as he hit just 23 home runs while batting .237.

Jackson hit a memorable home run in the 1971 All-Star Game at Tiger Stadium in Detroit. Batting for the American League against Pittsburgh Pirates pitcher Dock Ellis, the ball he hit soared above the right-field stands, striking the transformer of a light standard on the right field roof. In 1984, he would hit a home run over that roof.