Thirteen times he was ranked in the Top Ten in the U.S. between 1942 and 1966. In 1951 Seixas was ranked No. 4 in the world, two spots below Dick Savitt, while he was No. 1 in the U.S. ranking, one spot ahead of Savitt. In 1953, Seixas was ranked No. 3 in the world by Lance Tingay, and was also cited as being the World No. 1 in newspaper Reading Eagle the same year.
He is currently the oldest living male Grand Slam singles champion.
In a very long career, Seixas won scores of singles, doubles, and mixed doubles titles. His career was interrupted for three years by World War II, during which he served as a pilot in the United States Army Air Forces. He also became an All-American during his years at UNC.
He was also a great doubles and mixed doubles player, in which his major victories include: four consecutive mixed doubles crowns at Wimbledon from 1953–56, the first three with Doris Hart and the fourth with Shirley Fry; the U.S. National mixed doubles from 1953–55, all with Doris Hart; the U.S. National doubles in 1952 with Mervyn Rose and again in 1954 with Tony Trabert; the French National (French Open) doubles in 1954 and 1955, both with Trabert; the French National mixed doubles in 1953, with Doris Hart; and the Australian National (Australian Open) doubles in 1955, with Trabert.
In 1966, Seixas was rated as the Senior Squash Champion of America.
Seixas and Trabert won the Davis Cup in 1954, against Australia. Seixas is rated fifth in the category of Most Davis Cup Singles matches (24), just behind Bill Tilden (25) and Arthur Ashe (27). He served three times as Captain of the US Davis Cup team. He was 38–17 lifetime in Davis Cup matches.
Seixas was a stockbroker from the late 1950s until the early 1970s. Afterwards he worked as a tennis director for a resort in West Virginia and at a Hilton Hotel in New Orleans. He moved to California in 1989 where he established a tennis program at the Harbor Point Racquet and Beach Club. In 1998, unable to play tennis any longer due to his knees, he chose to become a bartender.