Mads Roerslev started out at FC Copenhagen's Academy, making his senior debut in the Danish Cup in 2016.

His debut came with a goal during a 6-1 win over Jammerbugt. He would make four more appearances that season and signed a new contract in the summer. He spent some time on loan at Halmstads BK and then Vendsyssel FF in the following seasons, making a handful of appearances for both.

In August 2019, he joined Brentford's B team on a two-year contract. He broke into the first-team squad that season, playing 10 games in his debut campaign and earning a new contract. Promoted to the first team that summer, he played back-up to Henrik Dalsgaard during Brentford's charge to the play-off final, which they lost 2-1 to Fulham.

“[Dalsgaard] had a huge impact on me,” said Roerslev.

“From our first training session together, he shared his knowledge with me. He’s a great defender and I really wanted to learn from him. We would analyse things together. He’s a really good guy and was a role model at Brentford for the young players in the squad.”

The following season, a foot injury ruled Roerslev out for four months. But an injury to Dalsgaard towards the end of the campaign allowed him to claim a spot in the starting XI, a place he kept through to the 2021 Championship play-off final. During a 2-0 victory over Swansea City, he produced a quality assist for the second goal by Emiliano Marcondes.

"To make the assist for the second goal was a feeling I’ll never forget – there’s nothing that compares,” he recalled.

"It was crazy. I remember facetiming my family on the pitch after the game. Everyone was so happy. It was one of the greatest days of my life.”

In his and the club’s first season in the Premier League, Roerslev played 26 times scoring his first Premier League goal against Aston Villa to win the game 2-1.

The following season injuries to Aaron Hickey and Kristoffer Ajer saw Roerslev get another chance to impress in the Premier League. He played 20 games as the Bees finished ninth.

Roerslev then played 34 games in the Premier League, which was a career high, and scored in a 2-2 draw with Chelsea in March.

Roerslev has represented Denmark up to under-21 level and has been involved in senior squads.