Dennis attended Churchland High School and then Old Dominion University in Norfolk, Virginia, Riddleberger was drafted by the Pittsburgh Pirates in 1967.

He was 24–16 with a 3.22 earned run average over four seasons in their farm system when he was traded to the Washington Senators for George Brunet on August 31, 1970.

[1] Riddleberger spent the entire 1971 season in the majors, but after getting off to a slow start (11.12 ERA in April), found himself doing mostly mop-up duty.

[2] Shortly after the Senators moved to the Dallas–Fort Worth metroplex and were rebranded as the Texas Rangers, Riddleberger was traded along with Del Unser, Terry Ley and Gary Jones to the Indians for Roy Foster, Rich Hand, Mike Paul and Ken Suarez at the Winter Meetings on December 2, 1971.

[3] Despite a below league average 2.50 ERA in 54 innings pitched in 1972, Riddleberger again found himself in mop up duty (the Indians were 4-34 in games in which he appeared).