Brown began playing rugby when he was in Primary 3 at Watsonian FC.

Brown worked up the age grades of the club alongside playing for his school George Watson’s College. In 2023, he made three appearances for the Watsonians FOSROC Super Series team in the Sprint.

He was named as one of the fifteen FOSROC Scottish Rugby contracted academy players aligned to Edinburgh Rugby for the 2023/24 season.

Previously, Brown captained Scotland at the 2023 U18 Six Nations festival, and made his Scotland U20 debut against Ireland in the 2024 U20 Six Nations. He was selected for Scotland A against Chile in November 2024, coming off the bench in the second half.

Jack was also a competitive swimmer and represented Heart of Midlothian Swim Club at national level. He decided to stop swimming during the Covid-19 pandemic lockdown to prioritise rugby.

Brown was a standout performer at the tail end of the 2024/25 season, showcasing maturity in huge home fixtures against Lions and Sharks.

Brown made his first professional start that same season, away to Zebre Parma at Stadio Sergio Lanfranchi.