Tizard began playing rugby from the age of seven at Guildfordians RFC, before joining the youth system at Harlequins when he was 16 years old, and also earning a scholarship to attend Cranleigh School.[3] He made his senior debut in October 2019, aged 19, during the pool stage of the 2019–20 Premiership Rugby Cup, then spent the rest of the season on loan at RFU Championship side London Scottish.[4] He became a regular player in the Harlequins first team over the course of the 2021–22 Premiership season.[5]
Tizard made his competitive debut for Saracens against former club Harlequins in September 2022, playing the full 80 minutes of the match to aid his new team towards a 30–27 win.[8] Throughout the season, Tizard established himself as a first-choice player, starting the majority of their Premiership games, as well as all of their fixtures in the 2022–23 European Rugby Champions Cup.[4] On 27 May 2023, he won his first Premiership title, as he started in the final and helped Saracens to achieve a 35–25 victory over Sale Sharks.[9]
In September 2022, Tizard received his first call-up to the England senior squad, ahead of the Autumn international series, although he has not yet made his test debut.[10]
During his childhood, Tizard's favourite Premiership Rugby team was Wasps, and he regularly attended matches when the club was based at Adams Park. He cites England second-row forward Joe Launchbury as the biggest influence on his own career.[11]