As a young man, Yablonski married Ann (née Huffman). Their son Kenneth J. Yablonski was born in 1934. Yablonski married again, to Margaret Rita (née Wasicek), an amateur playwright. They had two children, Joseph "Chip" (b. 1941) and Charlotte Yablonski, b. 1944. Both sons became labor attorneys, representing their father in his union activities and later in private practices. Charlotte became a social worker in Clarksville, Pennsylvania, where her family lived. She took leave to work in 1969 on her father's campaign for the UMWA presidency.