While at Wilbur Cross High School, Williamson played on the Connecticut state champion teams of 1966–67, 1967–68 and was runner up to crosstown rival Hillhouse in 1968–69, losing 76-71 in the final. For his senior year, Williamson led the nation in scoring with a gaudy 38.7 points per game average. His team was upset in a semi-final match against Bridgeport Central, with an ending score of 105-103. Cross previously won over Central in the 1968 title game in a 123-82 game. Williamson's teammates, Alex Scott, and Clint Davis, led the way in the romp for Cross's third consecutive title. Williamson chipped in 17, which was his average that year.
College career
Williamson went on to team up with Scott and Davis at New Mexico State University where he averaged 27 points per game his sophomore and junior years. In the final regular season game of 69-70 Cross played St. Anthony's Catholic of Washington, D.C.. They were the no. 1 team in the DC metro area and one of the top five in the country. Coached by Georgetown's John Thompson, Cross was a huge underdog. All-America player Williamson scored 36 points and fellow all-stater Danny Hardy had 22 to pace the Governors to a 74-66 win. In that game, John Thompson took his team off the floor and to the locker room with about two and a half minutes remaining in protest over officiating, and not return. During this season, Williamson had a string of nine straight games in which he scored 40 or more points.
Professional career
As a rookie, Williamson landed a roster spot with the New York Nets of the ABA as a free agent for the 1973–74 season. Before signing with the ABA Nets, he had been eligible for the NBA draft that year, being selected by the Atlanta Hawks in the 6th round with the 96th pick, but didn't make the cut for their roster. Despite being a rookie, Williamson quickly entered the New York starting lineup along with superstar Julius Erving and rising star rookie Larry Kenon. After Williamson became a starter, the team's fortunes quickly turned for the better and the team ended up winning the ABA Championship that season. Williamson was named to the 1974 ABA All-Rookie team. Continuing with the Nets, Williamson, in Game 6 of the 1976 ABA Finals, scored 28 points with 16 of them coming in the fourth quarter; both him and Julius Erving offensively carried the team, combining for 59 points leading the Nets to come back from 22 points behind to win the game, the series and the ABA championship. In the 1976–77 season, Williamson was traded to the Indiana Pacers midseason; in the middle of the next season he was traded back to the Nets. In the 1979–80 season he was traded from the Nets to the Washington Bullets which would be the last team he played for before the end of his NBA career until the 1980–81 season. By this time Williamson had completely declined as a player despite being only 29 years old and wouldn't return to the NBA after that season due to contract disagreements in free agency and not maintaining a healthy weight to play. In 1982, he would play for the CBALas Vegas Silvers in an attempt to get recruited back into the NBA but never received any offers. In his ABA/NBA career Williamson scored 9,017 points. He averaged between 11.5 and 29.5 points in every ABA/NBA season except for his last. Williamson still holds Nets team records in various categories, including most free throw attempts in a game, with 24. Williamson's jersey number was retired by the New York/New Jersey Nets franchise on December 7, 1990; Williamson is one of four players who were with the Nets during their ABA days with a retired number; the other three include Wendell Ladner, Bill Melchionni, and Julius Erving.
