At the end of the 2020–21 season, Frank became only the second Brentford head coach or manager to achieve promotion to the top-flight of English football.

[17][18] Frank resigned on 9 March 2016 after receiving criticism from chairman Jan Bech Andersen, under a pseudonym, on an online supporters' forum.

[19][20][21] On 9 December 2016, Frank moved to England to join Championship club Brentford as assistant head coach alongside Richard O'Kelly.

[27] He took over a club rocked by the recent death of technical director Robert Rowan and endured a tough start to his tenure, winning just one of his first 10 games,[28] before stabilising the team's form after a change to a 3–4–3 formation.

[28] After an uneven start to the 2019–20 season and switching back to a 4–3–3 formation,[30][31] 10 points from five matches in October 2019 saw Frank nominated for the Championship Manager of the Month award.

[38] In the midst of a 21-match unbeaten run in league matches,[39] five wins in December 2020 won Frank the Championship Manager of the Month award.

[40] Following a run to the club's first ever appearance in the semi-finals of the EFL Cup, Frank managed Brentford to a second-successive third-place Championship finish during the 2020–21 regular season.

[44][45] Halfway through the 2021–22 season – with Brentford placed 14th in the Premier League table, 9 points clear of the relegation zone – Frank and his assistant Brian Riemer signed 3+1⁄2-year contract extensions on 21 January 2022.

[51] With 10% less possession per match compared to the previous season in the Championship, Frank and the coaching staff adjusted the team's style of play, emphasising scoring from dead-ball situations and counter attacks.

[58] Brentford's Premier League status was assured with four matches to play, but despite the low placing,[59] the club ended the season 13 points above the relegation zone.