Williams grew up in South-Central Los Angeles. His parents were immigrants from Belize. Williams played football with his cousins growing up, but his football career was ended by injury and the disapproval of his mother. Williams's father was an amateur bike racer, and Williams took up riding to try to connect with his father. Williams's first bike ride was planned to be 70 miles along the Pacific Coast Highway but it ended when he cramped up after 50 miles. His father rode away and left Williams on the side of the road where his aunt picked him up. Williams understood the message his father was trying to convey: "Racing bikes is hard and you need to be serious about it." He began racing a few months later. Williams knew there were not a lot of opportunities for black men in South-Central and saw cycling as a means to avoid getting in trouble with the law. Williams did well in the sport, winning many Criteriums in California as a teen. His goal was to join the US National Team, but felt that they ignored him despite his racing results. Williams eventually made the team after winning the 2006 Junior Track National Championship in Keirin. The National Team wanted Williams to focus on track cycling but he loved road racing and wanted to continue training for that. As a teen, Williams's father had introduced him to racer Rahsaan Bahati, and was from Bahati that Williams realized he could go professional and race in Europe. Williams joined the Rock Racing team in 2007—the same team his mentor Bahati raced for—while still racing with the national team. Rock Racing folded in 2009 and Williams moved to Axel Merckx’s Trek-Livestrong development team for the 2010 racing season. In 2010, Williams went to Europe where he had success in Kermesse races in Belgium. He was a domestique for Taylor Phinney in the 2010 Paris–Roubaix Espoirs race that Phinney won. Although the national team coaches felt that Williams had a lot of potential, Williams decided not to go back to Europe after flying home to visit his family during the 2010 season. Williams had developed a reputation for being "hard to deal with", but he contends it was in part due to being a young sprinter who needed guidance and part due to him being stereotyped as an "angry black man". With his professional career seemingly ended when he left the national team, Williams moved back to California and attended Moorpark College. He raced for a few low level teams during this time, but did not have much success. William's younger brother, Cory Williams, joined Cylance Pro Cycling and convinced them to hire Justin Williams as well. Williams was hesitant to get back into professional racing, but saw it as a good opportunity to support his brother. Despite his initial trepidation, Williams had a breakout season, winning 15 races. Cylance promised Williams they would keep Cory on the team, but they cut him after one year. Williams was contractually obligated to continue racing, and although he debated sandbagging he decided that "I don't win for them, I win for me" and had another stellar season, racking up 14 wins. In 2018, Williams signed with Specialized-Rocket Espresso fixie criterium racing team at the Red Hook Crit. He felt instantly at home with the other team members, and appreciated the lack of politics and drama that he had tired of on the UCI circuit. The team allowed Williams to compete as an independent rider in road races as well. Williams won both the road race and criterium national championships in 2018. He placed in the top 3 in 30 of the 35 races he rode that year. In 2019, Williams founded his own team, L39ion of Los Angeles, where he is both the manager and primary sprinter. The team is a mix of profession cyclists and development riders, many from South-Central LA, as well as both of Williams's brothers. The "39" in the name represents 39th street where Williams grew up. Williams once again won the criterium national championship. Williams found racing at the pro level difficult because he was the only minority, and that made him want to make the sport of cycling "great for everyone". This experience led him to work for increased inclusion and innovation in cycling. As part of that effort, he mentors African American and Hispanic riders on his development team, Endo CNCPT.
