Shimkus is a lifelong resident of Collinsville, part of the Metro East portion of the St. Louis metropolitan area. He is the son of Kathleen N. (née Mondy) and Gene L. Shimkus. His paternal grandfather was of Lithuanian descent. While in the Army, he earned the Expert Infantry Badge, Ranger tab, and Parachutist Badge. He served overseas with the 54th Infantry Regiment in West Germany.