He made his first team debut on 7 September 2011 in a Danish Cup match against Holstebro, a game Horsens won 5–0.

[4] As a consequence, on 5 July 2013, Rønnow moved to Superliga club Esbjerg fB on a season-long loan following the departure of Lukáš Hrádecký.

In the 2016–17 season, Rønnow played seven games in the Europa League qualifiers in which he was sent off once, as Brøndby were eliminated by Panathinaikos.

[8] He also played three times in the Danish Cup tournament and appeared in 34 total matches in regular season as well as the championship round.

Due to insecure performances, Eintracht loaned Kevin Trapp from Paris Saint-Germain shortly before the end of the summer transfer window, and Rønnow lost his starting spot.

[21][22] On 20 July 2021, Rønnow joined Union Berlin on a three-year contract,[23] and would initially start his stint with the club as a backup to starter Andreas Luthe.