Rotterdam (2003–2007) Slagter played his first seasons for
Rotterdam Basketbal. In 2006 he was named the
MVP under 23.
West-Brabant Giants (2007–2010) Slagter left Rotterdam in 2007 for
WCAA Giants in
Bergen op Zoom. In 2010 he reached the DBL Finals with the Giants.
ZZ Leiden (2010–2013) After the 2009–10 season Slagter went to
Leiden to play for
Zorg en Zekerheid Leiden. In three seasons with Leiden he won the DBL twice, the
Dutch Supercup twice and the
NBB Cup once.
GasTerra Flames (2013–2014) Slagter left the defending champion Leiden and signed with
GasTerra Flames (formerly Donar) from
Groningen for the 2013–14 season. He won the
NBB Cup with the Flames and got the top seed in the DBL. Slagter averaged 8.8 points and 4.4 assists in 24.6 minutes per game in the DBL regular season. He also was the league leader in three point percentage, as he was shooting .500 from behind the arc. On 21 April 2014 Slagter was named the best player in the Netherlands, by receiving the
DBL Most Valuable Player Award. Slagter was the first Dutch MVP since
Peter van Paassen in 2009. After the Playoffs, in which Flames beat Amsterdam, Leiden and Den Bosch Slagter was named
Playoffs MVP. He averaged 8.9 points, 3.1 rebounds and 2.8 assists in 14 playoff games.
Den Bosch (2014–2016) On 8 July 2014, Slagter signed a 2-year contract with
SPM Shoeters Den Bosch.
Donar (2016–2019) On 8 July 2016, Slagter signed a 1-year contract with Donar, which he previously played for in the 2013–14 season, when the team was named "GasTerra Flames". In the 2016–17 season, Slagter was named
DBL Sixth Man of the Year. In April 2017, Slagter extended his contract with 2 more years. In the 2017–18 season, Slagter won his second Sixth Man of the Year Award. ==3x3 basketball==