 Germinal Beerschot, but joined the AFC Ajax youth academy at age 15 like compatriot Thomas Vermaelen a year before.He made his first-team debut on 25 January 2004 against NEC Nijmegen, but appeared only eight times in the Eredivisie in his first two years combined.After a solid 2005–06 (spent on loan) at Vitesse Arnhem, De Mul returned to Amsterdam and figured prominently during his first full year, starting in 23 of the 28 matches he appeared in.