After finishing his playing career, Seymour continued a successful coaching career in the NBA, coaching three different teams. Altogether he coached four different teams in eight seasons. In 1961, he was the head coach of the Western Division Team in the All Star Game.
Seymour was mentioned in the ESPN documentary, Black Magic, which told the story of African-Americans and basketball. In a segment about Cleo Hill, it was revealed that during the 1961–62 season, Bob Pettit and Cliff Hagan approached management and complained that Hill was taking too many shots. (Allegedly, this was just a cover-up for their desire to not play with an African-American teammate.) Management granted their wish, telling Seymour to severely diminish Hill's offensive role. Seymour refused and was fired 14 games into the season.
While coaching at Baltimore during the 1965-1966 season, Seymour deliberately ended Johnny Kerr's then-record consecutive-games-played streak of 844 games by benching the team captain for one game. According to Kerr, only after the game did Seymour tell Kerr about his intention to end Kerr's streak, saying, "This will take the pressure off you." 
Seymour was featured in the book, Basketball History in Syracuse, Hoops Roots by author Mark Allen Baker published by The History Press in 2010. The book is an introduction to professional basketball in Syracuse and includes teams like (Vic Hanson's) All-Americans, the Syracuse Reds and the Syracuse Nationals (1946–1963).
Seymour still shares, with former teammate Red Rocha, the NBA record for most minutes in a playoff game with 67.