Mitchell became secretary-manager of Blackburn Rovers in 1884. The club then went on to five FA Cup wins in eight seasons between 1884 and 1891. The final win in 1891 was against the Notts County side featuring David Calderhead. It does not appear documented if Mitchell was on board for the first of the FA Cup finals. Even with 4 FA Cups, he remains the most successful major trophy winner in the history of Blackburn Rovers.
Mitchell left Blackburn after 12 years as manager in 1896.
Mitchell joined Woolwich Arsenal, after the club decided to appoint a professional manager following their defeat at the hands of Millwall in the FA Cup. There is some doubt over whether he was Arsenal's first- or second-ever manager; Sam Hollis is credited on the club's official website as being the club's first manager, other historical sources claim Hollis was only the club's trainer and thus Mitchell was the club's first manager – a view reinforced by the club's own official history, amongst other sources.
Regardless of whether Hollis was properly in charge of the club during his time there, Mitchell is nevertheless credited as being Arsenal's first professional manager. However, he only lasted less than a season in the job, and although the club would reach fifth in the Second Division in 1897-98, neither he nor most of his signings stayed long with the club. Mitchell resigned as Arsenal manager on 10 March 1898.
^Joy, Bernard (1952). Forward Arsenal!. Phoenix House. p. 11. "Arsenal's trainer, Hollis, however, was true to the nineteenth century pattern."
^Soar, Phil & Tyler, Martin (2005). The Official Illustrated History of Arsenal. Hamlyn. p. 30. ISBN0-600-61344-5. "A disastrous FA Cup defeat by non-League Millwall (2-4 away) on 16 January 1896 proved one turning point for the committee. They decided to appoint a secretary-manager, one T.B. Mitchell from Blackburn,..."
^Roper, Alan (2004). The Real Arsenal Story: In the Days of Gog. Wherry. p. 120. ISBN0-9546259-1-9. "After their humiliating defeat the Arsenal FC directors decided to appoint their first professional manager, and the first to be signed was Thomas Brown Mitchell, a Scot."
^Joy (1952). Forward Arsenal!. p. 12. "The first manager was T.B. Mitchell, from Blackburn Rovers and he was succeeded after a few months by G. Elcoat of Stockton."