Canseco and his fraternal twin Oswaldo were born in Havana, Cuba, on July 2, 1964, to Jose and Barbara Canseco. In December of 1965, the family, which included an older sister, left Cuba and settled in Opa-Locka, Florida. At Coral Park High School, Canseco was a talented, if somewhat slight of frame ball player who did not make the varsity team until his senior year. A scout for the Oakland A's, a fellow Cuban, discovered him, and he was a 15th-round draft pick in 1982. He first played for the Rookie League or farm teams in several states before making his major-league debut in September of 1985 in an A's game against Baltimore.
By 1986, Canseco's impressive hitting power had earned him the American League (AL) Rookie of the
Year title. Two years later, he declared his intention to set a baseball first, the 40-40, and did it on September 23, 1988, in an A's game against the Milwaukee Brewers. He was named the AL's Most Valuable Player (MVP) that season, and often earned comparisons to Reggie Jackson, who praised his talents. Canseco was famous for his at-bat twitches, but the quirk only seemed a warm-up to the real stunt: soaring home runs. At Game 4 of the American League Championships in Toronto in 1989, Canseco hit the ball into the fifth deck of the vast new Toronto Sky Dome that estimates pegged as a 540-foot hit.