Seminick was born in Pierce, West Virginia to Lemko immigrant parents, but moved to Muse, Pennsylvania when Andy was two. His father was a Rusyn from Żegiestów in Nowy Sacz County. He was contracted as an amateur free agent in 1941 by the Pittsburgh Pirates. Seminick led the Appalachian League in 1942 with 15 home runs and 202 total bases, and was among the league leaders in batting average. In , he had a .303 batting average with the Knoxville Smokies of the Class-A Southern Association, and was purchased by the Philadelphia Phillies.