On 11 January, Williams began training with the All Blacks Sevens team. However, his introduction to the sport and national team began at the 2015 Dubai Sevens, as a guest of the tournament, where he also witnessed the debut of his sister, Niall, for the New Zealand women's team. His inclusion in the international sevens arena has been "Touted as the biggest gain from the 15-a-side ranks". On 25 January, Williams was named in the squad for the 2016 Wellington Sevens. On 30 January, he marked his international debut against Russia, scoring a try with his first touch of the ball. He went on to play in the remainder of New Zealand's pool matches. The following day, Williams played in the quarter-final against Kenya, scored a try in the semi-final against England and played in the final's victory against South Africa. After Wellington, he played in the following 2016 Sydney Sevens, including the victory against Australia in the final. He was rested for the subsequent 2016 Las Vegas Sevens and was then ruled out of the 2016 Canada Sevens due to a swollen knee. Williams returned from injury against France in the 2016 Hong Kong Sevens and scored a try in the final pool stage game against Samoa. He went on to play in the quarter-final against Wales, the semi-final against South Africa and the loss against Fiji in the final. The following week he played in the 2016 Singapore Sevens, losing to Samoa in the second-tier Plate final. He went on to play in the 2016 Paris Sevens; however, his tournament was cut short by a knee injury. The following week he played in the 2016 London Sevens, the final tournament of the World Series, scoring a try against Russia. His tournament was again cut short, by an ankle injury early in the quarter-final loss to the United States, with New Zealand finishing the World Series in third place.