The son of two life-long squash coaches, Christina and Bob Brownell, Timmy grew up playing squash from an early age with his sister, Jessy. Timmy excelled in junior squash, rising up the ranks and cementing himself as one of the top players in the country. 2014 proved to be a banner year from Timmy where he won the 2014 U19 U.S. Junior Open title. Brownell represented Team USA at the 2014 World Juniors where he recorded a perfect 5-0 record in the team competition, earning him the 2014 USOPC Athlete of the Year Award. Timmy led his high school, Belmont Hill, to its first and only High School Nationals finals appearance in 2015.

Timmy matriculated to Harvard where he won back-to-back national team titles with the Crimson in 2019 and 2020 and served as captain for two years. Over the course of his tenure, Brownell was a two-time All-American and two-time All-Ivy.

Shortly after graduating Harvard in 2020, Timmy joined the US Squash High Performance Program and based himself at the Arlen Specter US Squash Center in Philadelphia in 2021. Over the course of his first full-time season on the PSA World Tour in 2021-2022, Timmy ascended from world No. 117 to world No. 69. Timmy ended his first full-time season with a flourish, winning his first title at the ToC Challenger in Grand Central Terminal in May, followed by his first S.L. Green U.S. Championship title in June.

Brownell carried the positive momentum into his second season, capturing two titles in the fall of 2022 in Lousville and Vancouver and reaching a new career high ranking of world No. 58 in November.