Paepke was born in Long Beach, California, and graduated from Garden Grove's Rancho Alamitos High School in 1963.

Dennis spent five full years in the Angel farm system until he was traded to the Royals, a first-year expansion team, in December 1968 along with catcher-outfielder Ed Kirkpatrick for Baseball Hall of Fame pitcher Hoyt Wilhelm.

Paepke had a strong year offensively at Triple-A Omaha in 1969, and was recalled by the Royals for his first taste of MLB action in June.

After spending all of 1970 with Omaha, Paepke returned to Kansas City for 1971, getting into 60 games and belting his two career MLB home runs (off 20-game-winners and American League All-Stars Wilbur Wood and Mickey Lolich).

In 1974, Paepke suffered a career-shortening injury when he was spiked by a player with sharpened cleats, tearing ligaments in his left arm.