Aker was originally signed as an outfielder by the Kansas City Athletics and led the Nebraska State League in stolen bases in his first year, 1959, before being converted to a pitcher before the 1960 season.

[1] Aker made his major league debut for the Athletics on May 3, 1964, tossing 3+1⁄3 innings in relief while allowing three earned runs in an 8–7 win over the Minnesota Twins.

[5] On April 24, 1968, in just the eighth baseball game ever played at the Oakland–Alameda County Coliseum, Aker pitched the last five innings, allowing no runs and earning the win in an 11-inning victory over the New York Yankees.

[1][11] Aker pitched almost three seasons in the NL, and was standing in the bullpen feet away from where teammate Hank Aaron's historic 715th home run landed on April 8, 1974.

[13] In 1997, he was honored by President Bill Clinton with a "Giant Steps Award" for his work teaching at-risk Native American children on reservations in Arizona and New Mexico.