Williamson played high school basketball at Wilbur Cross High School in New Haven, Connecticut and played college basketball at New Mexico State University. He was a 6'2" guard. Williamson was drafted by the Atlanta Hawks in the sixth round (96th pick overall) of the 1973 NBA Draft but had other plans.
Williamson helped to lead Wilbur Cross to the Connecticut state championship in 1967, 1968 and was runner up to crosstown rival Hillhouse in 1969, losing 76-71 in the final. For his senior (1969-70) year, Williamson led the nation in scoring with a gaudy 38.7 points PPG. His team was upset in a semi-final match that is still talked about, 105-103 to Bridgeport Central. Cross had destroyed Central in the 1968 title game by 40 points in a 123-82 pasting of a heralded Bridgeport squad that boasted it would defeat the Governors.
In the final regular season game of 1969-70 Cross played St. Anthony's Catholic of Washington, DC, the no. 1 team in the DC metro area and one of the top five in the country. Coached by Georgetown University legend John Thompson, Jr., 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. The game was memorable in that John Thompson took his team off the floor and to the locker room with about two and a half minutes remaining to protest the officiating. They did not return. Anyone from New Haven who was at the game will tell you, the real reason was John "SuperJohn" Williamson! That was the way Thompson kept "SuperJohn" from scoring forty or more points. He was not the only coach who made that boast. For as long as basketball is played in New Haven, the John Williamson legend will live on. Of note is that Williamson had a string of nine straight games in which he scored 40 or more points.
He went on to team up with Scott and Davis at New Mexico State University, where he averaged 27 PPG his sophomore and junior years.
As a pro rookie, Williamson landed with the New York Nets of the ABA as a free agent for the 1973-74 season. Williamson quickly entered the New York starting lineup, despite being a rookie. 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 had a remarkable performance in Game 6 of the 1976 ABA Finals. He scored 28 points, with 16 of them coming in the fourth quarter, leading the Nets to come back from 22 points behind to win the game, the series and the ABA championship.
Williamson's jersey number (23) was retired by the New York/New Jersey Nets franchise on December 7, 1990, only one of four players who were with the Nets during their ABA days who had their numbers retired, including Wendell Ladner, Bill Melchionni, and Julius Erving.
In 1977 Williamson was traded to the Indiana Pacers in midseason; in the middle of the next season he went back to the Nets. In the 1980 season he went from the Nets to the Washington Bullets where he finished his eight year career during the 1981 season.
In his pro career Williamson scored 9,017 points. He averaged between 11.5 and 29.5 points in every season except for his last.