During the 1920–21 English football season, Brentford competed in the Football League Third Division. It was Brentford's inaugural season in the Football League and ended with the club successfully applying for re-election.

Brentford competed in the First Division of the Southern League during the 1919–20 season and were named as founder members of the new Football League Third Division for 1920–21. In preparation, 11 new players were signed and £2,000 was spent on improvements to Griffin Park prior to the beginning of the season. Despite flirting with a position in mid-table during August and September 1920, the club endured a torrid season, sinking to the re-election places by October and largely remaining there for the rest of the season, in addition to exiting the FA Cup in the first round. Brentford successfully applied for re-election to the Football League at the end of the season without going to a poll. Three directors resigned after the season, due to a deficit of nearly £6,000.

The goalscoring of former Arsenal forward Harry King provided one of the rare bright spots of the season, with his 18 goals accounting for nearly half that of the team's total. King also became the first Brentford player to register a Football League hat-trick in a 5–0 thrashing of Grimsby Town on 28 March 1921, the Bees' biggest win of the season. The result established the club record for the highest winning margin in a Football League match, which would stand until broken in September 1929. Secretary manager Fred Halliday stood down after the season and reverted to an administrative role within the club, with management advisor Billy Brawn also stepping down. Brentford's 9 league victories during the season is the joint-fewest in the club's history and Jimmy Hodson became the club's record-oldest player when he appeared in the final match of the season at age 40 years, 8 months and two days.

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