A hard-throwing fastballer, Dennis recorded a 1.47 earned run average for the Triple-A Jacksonville Suns in 1965 before being called up to the Cardinals in the midseason. He ended the year with a 2–3 mark, six saves, and a 2.29 ERA in 41 appearances, on a team whose collective ERA was 3.77. His efforts gained him St. Louis Rookie of the Year honors. The next year he went 4–2 in 38 games, including one emergency start, but his ERA dipped to 4.98. He has the distinction of winning the first game in Busch Stadium II on May 12, 1966. Before the 1967 season, he was sent by St. Louis along with Walt Williams to the Chicago White Sox in exchange for Johnny Romano and a minor leaguer, but he never appeared in the majors for Chicago.
