Dave was a right-handed outfielder who starred for Florissant Valley Community College and Maryville College. A top prospect, he was scouted by several Major League Baseball teams and in 1983, he was invited to try out for the St. Louis Cardinals at Busch Stadium.

In the late 1970s, Slemmer played for McCluer High School and the Ferguson American Legion team.

In 1981-82, Dave was an outfielder at Flo Valley, where he was selected to the All-Conference team that second season after batting .381 with 10 home runs and 55 RBIs. In 1983, he led Maryville in doubles, runs scored, total bases and games started.

From 1983-92, Slemmer was an outfielder for the Sotolar Pointers in the CYC Major League. During that time, the Pointers won eight league championships and seven NBC state titles. Slemmer drove in the winning run in the 1984 NBC state title game. In 1987 and 1991, the Pointers finished in the top 10 at the NBC tournament in Wichita. Slemmer played in six CYC Major all-star games.

He was a power hitter who consistently hit well over .300 and was a solid, strong-armed defensive outfielder.