To gain experience, Mulder joined Excelsior on loan for the season 2008–09 under manager Ton Lokhoff.
Excelsior had just been relegated to the Eerste Divisie and were in need of a first team goalkeeper after Ronald Graafland left the club to join Vitesse Arnhem.

On 8 August 2008, Mulder made his debut for Excelsior in a 1–0 loss against Telstar.
Since making his debut for the club, he had established himself as the first choice goalkeeper at Excelsior.
On 6 March 2009, Mulder received a straight red card, just a few minutes before half-time, for a professional foul in the penalty box, resulting in a penalty to FC Oss.
After being sent off, Arjan van Dijk replaced him in goal.
van Dijk could not save the penalty.
The match finished with FC Oss beating Excelsior 3–1, in a game that saw three penalties awarded, (two for the home side and one to Excelsior).
After a two-match ban because of his sending off, Mulder made his return in goal on 27 March 2009 against SC Cambuur in a 1–1 draw.
At Excelsior, he played 36 matches for the third club from Rotterdam, finishing in fifth place in the Eerste Divisie.

His performance was strong at Excelsior that he was the only player in the Eerste Divisie selected by Johan Neeskens for the Netherlands B squad.
To gain experience, Mulder joined Excelsior on loan for the season 2008–09 under manager Ton Lokhoff. Excelsior had just been relegated to the Eerste Divisie and were in need of a first team goalkeeper after Ronald Graafland left the club to join Vitesse Arnhem.
On 8 August 2008, Mulder made his debut for Excelsior in a 1–0 loss against Telstar. Since making his debut for the club, he had established himself as the first choice goalkeeper at Excelsior. On 6 March 2009, Mulder received a straight red card, just a few minutes before half-time, for a professional foul in the penalty box, resulting in a penalty to FC Oss. After being sent off, Arjan van Dijk replaced him in goal. van Dijk could not save the penalty. The match finished with FC Oss beating Excelsior 3–1, in a game that saw three penalties awarded, (two for the home side and one to Excelsior). After a two-match ban because of his sending off, Mulder made his return in goal on 27 March 2009 against SC Cambuur in a 1–1 draw. At Excelsior, he played 36 matches for the third club from Rotterdam, finishing in fifth place in the Eerste Divisie.
His performance was strong at Excelsior that he was the only player in the Eerste Divisie selected by Johan Neeskens for the Netherlands B squad.
Erwin Mulder