Seiling played in the Toronto Maple Leafs junior system from 1960-61 through 1963-64. He won the Memorial Cup in 1960-61 with the St. Michael's Majors and in 1963-64 with the Toronto Marlboros. In 1963-64 he also played with the 1964 Canadian Olympic Hockey Team. He was also be traded to the New York Rangers in the Andy Bathgate trade.
Seiling would remain with the Rangers 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.
He also played for Team Canada in the 1972 Summit Series.
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.