Signed by the Toronto Maple Leafs in 1962, Seiling played only one game with the Leafs and spent most of his time in the minors. The next year he would participate with the Canadian hockey team that played in the 1964 Winter Olympics. He would also be traded to the New York Rangers where he would remain for the next decade, except for a brief time when he was claimed by the St. Louis Blues in the 1967 NHL Expansion Draft but was immediately traded back to the Rangers. His defensive abilities eventually helped guide the Rangers to the finals in 1972.
In 1974, he was claimed on waivers by the Washington Capitals, but after appearing in just one game with the Capitals he was traded to the Maple Leafs. Seiling would also play for the Blues and Atlanta Flames before retiring at the end of the 1978–79 NHL season. In 2006, Seiling was appointed Chair of the Ontario Racing Commission, a position he still holds today. In 2009, Seiling was ranked No. 41 on the all-time list of New York Rangers in the book 100 Ranger Greats (John Wiley & Sons).