Wilhelm made his professional debut with the Mooresville Moors of the Class-D North Carolina State League in 1942. He served in the United States Army in the European Theater during World War II and participated in the Battle of the Bulge, where he was wounded, earning the Purple Heart for his actions. He rose to the rank of staff sergeant while in the Army, and played his entire career with a piece of shrapnel lodged in his back as a result of the wounds he received in battle. Wilhem carried the nickname "Old Sarge" because of his service in the military.