Seixas was born in Philadelphia, Pennsylvania, to Anna Victoria (Moon), who was of Irish descent, and Elias Victor Seixas Sr., who was born in Brazil, of Portuguese Sephardi Jewish ancestry. He attended and graduated from the William Penn Charter School, where he was a tennis star.
After serving in World War II, he attended the University of North Carolina at Chapel Hill (UNC), where he was a member of Alpha Sigma of the Chi Psi fraternity. He graduated in 1949, the same year that UNC awarded him the Patterson Medal in athletics.
Thirteen times he was ranked in the Top Ten in the U.S. between 1942 and 1956. 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 the Reading Eagle newspaper the same year.
In a very long career, Seixas won scores of singles, doubles, and mixed doubles titles. He entered the U. S. Championships men's singles a record 28 times between 1940 and 1969. 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.
His major singles wins include Wimbledon in 1953 over Kurt Nielsen and the U.S. National (U.S. Open) in 1954 over Rex Hartwig.
He was also a successful doubles and mixed doubles player. In 1952 he won the U.S. National doubles with Mervyn Rose. In the mid-fifties he formed a successful partnership with Tony Trabert, winning the 1954 French and U.S. Championships as well as the 1955 Australian and French Championships. Additionally they won the decisive third point in the 1954 Davis Cup win over Australia. Seixas won 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; and the French Championships mixed doubles in 1953, with Doris Hart.
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 inducted into the International Tennis Hall of Fame in 1971.
He was inducted into the Blue Gray National Tennis Classic Hall of Fame.
Seixas was a stockbroker from the late 1950s until the early 1970s. Afterwards, he worked as a tennis director for the Greenbrier resort in White Sulphur Springs, 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 Mill Valley (Marin County). In 1998, unable to play tennis any longer due to his knees, he chose to become a bartender.
He is currently the oldest living male Grand Slam singles champion.