Sonck moved to Ajax in the summer of 2003, to replace departed striker Mido. He made his debut on the 12 August against Grazer AK. He scored his first goal for the club 13 September against RKC Waalwijk. Sonck never really made it in Amsterdam, partly because he was playing on the right wing a lot under coach Ronald Koeman. He moved to Borussia Mönchengladbach in the winter of 2004.
His game with Borussia Mönchengladbach of the German Bundesliga was hampered by injuries. Mönchengladbach then signed him on a permanent contract, a deal initially arranged already at the start of his loan. In the summer of 2005 Sonck had three of his ribs broken after a horrific tackle by Wilfred Bouma in a goalless, meaningless friendly between Borussia Mönchengladbach & PSV Eindhoven. Sonck took six months to recover, marking his return to competitive football with a goal in a 3-1 defeat by Bayern Munich. He scored three more in 13 further Bundesliga games for Mönchengladbach until he was forced out for three months with an injury in the hollow of his knee at the start of the season of 2006-2007.
He left Gladbach at the end of the 2006-07 season, joining Club Brugge on a year long loan deal while Borussia Mönchengladbach began playing in the second tier of the Bundesliga. Following the 2007-08 season, Sonck joined Brugge permanently for an undisclosed fee. At the end of the 2009/10 season, Sonck left Brugge to join Lierse S.K. on a free transfer having fallen out with manager Adrie Koster over contract negotiations and lack of first team action. Amongst his first goals for the club was an excellent overhead kick. Sonck was released in the summer of 2012 and spent a few months unemployed before joining newly promoted Waasland-Beveren near the end of October 2012.
Sonck was called for the national team during the 2010 World Cup qualifiers. In the match versus Spain he scored Belgium's only goal in that match, thus ending Casillas and Reina's undefeated streak of 710 minutes.