His performances caught the attention of fellow 1st Division club Sønderjyske, who signed him on 19 June 2023 on a three year contract.
[2] His first season with the club would prove to be successful, as Sønderjyske won the 1st Division title to gain promotion to the Superliga.
During their promotion campaign, Agger provided nine goals and five assists over 32 appearances in both the league and the Danish Cup.
[1] On 11 August 2024, Agger made his first appearance in the top flight of Danish football, coming on the 39th minute as a substitute as Sønderjyske fell 0–2 to Copenhagen at Sydbank Park.
[4][5] Mads is the younger brother of professional footballer Mikkel Agger,[1] who spent much of his career with Thisted before retiring in 2024.