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 pitching before the 1960 season. He made it to the majors as a side-arming sinkerballer, pitching for the Kansas City/Oakland Athletics, Seattle Pilots, New York Yankees, Chicago Cubs, Atlanta Braves, and New York Mets. During an 11-year baseball career, Aker compiled 47 wins, 404 strikeouts, a 3.28 earned run average, and 123 saves, an impressive total at the time. On September 10, 1965, Aker pitched 6.1 innings of relief, allowing just one run, to earn the W in a 10–5 win over the Baltimore Orioles His best season came in 1966 with the A's, when he led the American League in saves and games finished, had a 1.99 ERA, finished 13th in the MVP voting, and was named ALFireman of the Year by The Sporting News. On September 7, 1966, Aker earned save #30 for the season with 3.2 innings of shutout relief vs. the California Angels. On April 29, 1967, he pitched the last 8.1 innings of a 15-inning loss to the Boston Red Sox, striking out a career-high 8 and allowing just two runs. On April 24, 1968, Aker in just the eighth baseball game ever played at the Oakland–Alameda County Coliseum, pitched the last 5 innings, allowing no runs and earning the W, in an 11-inning win over the New York Yankees After a series of run-ins with A's owner Charles O. Finley, Aker, the team's union player representative, was made available in the expansion draft for the 1969 season, and was picked up by the Seattle Pilots. As one of their original players, on April 8, 1969, he earned a save in the first game in franchise history, a 4–3 win over the California Angels. His career got a second wind when he was traded to the Yankees that May, as he ran up a string of 33+ consecutive scoreless innings, still a regular season Yankee record. He led the Yankees in saves that year and finished both 1969 and 1970 with ERAs of 2.06, despite career-threatening back surgery in the intervening winter. He remained a mainstay of the Yankee bullpen until his trade to the Cubs in 1972, when New York's acquisition of Sparky Lyle from Boston made Aker expendable. 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. After his playing days ended, Aker managed in the minor leagues in 1975–85, winning the Governor's Cup with the 1982 Tidewater Tides, and was the Cleveland Indianspitching coach from late 1985 to 1987. He left pro baseball after the 1988 season to teach children, and for 20 years offered camps, clinics, and baseball instruction through his "Jack Aker Baseball" academy. In 1997, he was honored by President Clinton with a "Giant Steps Award" for coaching for his work teaching at-risk Native American children on reservations in Arizona and New Mexico. Aker is of Potawatomi ancestry.
