His most productive years came with Boston, where he won 24 games from 1956 to 1958, averaging 138 innings each season. After that, he appeared strictly as a reliever and saved a career-high 11 games for the Senators. In a seven-season career, Sisler posted a 38–44  record with a 4.33 ERA in 247 appearances, including 29 saves, 12 complete games, one shutout and 656⅓ innings. Sisler retired from baseball after the 1963 season to become an investment firm executive, a career that lasted for over 30 years, retiring as a vice-chairman for A. G. Edwards.