Tom completed his switch to Sunderland in June 2018.
The defender, who made 27 appearances in the Sky Bet Championship last season, signed a two-year deal at the Stadium of Light.
Flanagan became Jack Ross’ fourth signing of the summer, following Alim Ozturk, Jon McLaughlin and Chris Maguire to Wearside.
The 26-year-old started his career with Milton Keynes Dons, progressing through the ranks at Stadium MK, and captaining the under-18s, before making 45 first-team appearances for the club.
As a youngster, he spent time on loan with Kettering Town, Gillingham, Barnet, Stevenage and Plymouth Argyle before signing a permanent deal at the Pirelli Stadium in August 2015.
Flanagan played 20 times for Burton during their 2015-16 promotion winning season, turning out alongside fellow new recruit Jon McLaughlin.
He went on to play 75 league games for the Brewers and earned senior international recognition, with his Northern Ireland debut coming in June 2017 against New Zealand.