Thijs debuted at the international stage during the 2002 European Aquatics Championships in Berlin. Van Valkengoed's main achievement is winning the bronze medal in the 200 m breaststroke at the European Short Course Swimming Championships 2003 in Dublin, Ireland. Earlier that month he had qualified himself for the 2004 Summer Olympics with national records in 100 m and 200 m breaststroke. At the Olympics he was eliminated in the semi finals of the 100 m breaststroke, and in the heats of the 200 m breaststroke.
After his overtraining he decided to move back from Amsterdam to Lelystad and started to train with his sister with the goal to qualify themselves for the 2008 European Aquatics Championships in Eindhoven. At the end of 2007 both Thijs and Jolijn van Valkengoed reached their goal and qualified for the European Championship in their homecountry. During the European championships Thijs lowered his own national record at the 100 m breaststroke to 1:01.22 and instead qualified for his second Olympics. Later on his sister qualified herself for the medley relay at the 2008 Summer Olympics. Just days before the Games started, during the training-camp in Hong Kong, Thijs became ill. Due to the illness Thijs couldn't lower his national record again.