Rosen won his first national title in 1952, the National Men's Pairs in partnership with Arthur Grau. In August 1953, Rosen and Milton Ellenby made the pages of Life magazine as the youngest winners of the von Zedtwitz Life Master Pairs. (annual from 1930). The same year he won the McKenney Trophy which the American Contract Bridge League gives to the player earning the most masterpoints in a calendar year. Rosen won the Spingold tournament twice in a row, in 1953 and 1954, the latter helping him qualify for that year's Bermuda Bowl world team championship. The Bermuda win made him, at 25 years old, the youngest world champion ever to do so; a record broken by Bobby Levin in 1981 at the age of 23. Rosen's win earned him the title of World Bridge Federation World Life Master .
The September 1954 issue of Playboy Magazine the editors wrote, "If bridge has a world's champion, he's a twenty four year old guy named William Rosen." Rosen contributed a column on "Tournament Bridge" for the issue. He wrote, "The game holds a beguiling interest to most people because it is undoubtedly the most challenging of all card games, combining skill in bidding, play of the hand and also, the subtle art of gamesmanship."
In 1958, Rosen won the Lebhar Trophy for winning the Master Mixed Pairs at the North American Championships. He won the prize playing with his wife Eunice Rosen, also a champion bridge player. In 1965, Rosen was on the Chicago Team that captured the Sports Illustrated Trophy. Bridge guru Charles Goren called Rosen the "star performer of the Chicago team." 
In 1978 Rosen won the Grand National Teams. Rosen won his last national championship in 1980.
- In a 1983 column, Alan Truscott of The New York Times called Bill Rosen "[one] of the greatest names in the game".
- Bob Hamman, one of the world's greatest players, named Rosen as one of "three players that I'd call really intimidating – if you could see some obscure way declarer could work it out (to make his contract), you had to be afraid he would work it out."
Awards and honors
- McKenney Trophy, 1953
- ACBL Hall of Fame, von Zedtwitz Award 2014
- Bermuda Bowl (1) 1954
- North American Bridge Championships (8)
- "1953 LIFE Magazines For Sale". 2Neat Magazines (2neatmagazines.com). Retrieved 2014-12-16. Quote: "August 24, 1953 ... William Rosen and Milton Ellenby become youngest team to win Master Pairs Bridge tourney."
- , "bridgeguys.com" Archived November 9, 2011, at the Wayback Machine.
- Sports Illustrated, August 30, 1965, SportsIllustrated.com Archived December 16, 2014, at the Wayback Machine.
- "Induction by Year". Hall of Fame. ACBL. Retrieved 2014-11-10.
- "Rosen, William". Hall of Fame. ACBL. Retrieved 2014-12-16.
- "Bridge: Saying Farewell to the McKenney Trophy". Alan Truscott. The New York Times. June 5, 1983. Retrieved 2014-12-16.
- Manley, Brent, Editor; Horton, Mark, Co-Editor; Greenberg-Yarbro, Tracey, Co-Editor; Rigal, Barry, Co-Editor (2011). The Official Encyclopedia of Bridge (7th ed.). Horn Lake, MS: American Contract Bridge League. p. 53. ISBN 978-0-939460-99-1.