William began his professional esports career in 2017 when he joined the Dog Gaming squad. With this team, he earned a silver medal at the Insomnia60 tournament. He then moved to XENEX, but his performance there was lackluster. In March 2019, he transferred to London ES, where he quickly won gold medals at both Insomnia64 and Grosvenor Esports Season 2. That summer, the team secured a sponsor, which led to better results. They claimed victories in the WINNERS League Season 2: Europe Main Division and Insomnia65 Masters, as well as a second-place finish at ESL Premiership: Winter 2019.
In 2020, mezii joined Cloud9, but failed to make a significant impact with the team. After spending a year with Cloud9, he moved to Fnatic in 2021. There, William helped secure three major tournament wins: DreamHack Open November 2021, REPUBLEAGUE Season 2, and Elisa Masters Espoo 2022.
In 2023, mezii joined Team Vitality, where he is currently playing. With the team, he won BLAST Premier: Fall Final 2023, BLAST Premier: World Final 2023, and BLAST Premier: Fall Groups 2024. He also earned silver medals at ESL Pro League Season 19 and Intel Extreme Masters Dallas 2024, as well as 3rd-4th and 1st-4th placements at PGL Major Copenhagen 2024 and BLAST Premier: Fall Groups 2024, respectively.