Born in Herentals, Van den Broeck won the Junior World Time Trial Championship in 2001 and made his professional debut in 2003 as a stagiaire for the Quick-Step–Davitamon–Latexco team.
Rather than remaining in Belgium, however, Van den Broeck signed with the American U.S. Postal Service team of Lance Armstrong for his first full season as a professional in 2004. His only result of note that year was 6th-place overall in the Tour of Belgium.
Van den Broeck remained with Johan Bruyneel's team in 2005 when Discovery Channel took over the title sponsorship, and he continued with the Americans in 2006 after Armstrong retired. During that period he finished on the podium in stages of the Deutschland Tour and the Tour of Belgium (where he also won the mountains classification), and he rode to a top-20 finish overall in the Tour de Romandie. In June, however, Van den Broeck announced his intention to return to Belgium when he agreed to a two-year contract with the then-Davitamon squad, which would be re-branded in 2007 as Predictor–Lotto. The following month Van den Broeck scored his first win as a professional, taking the Schriek, Derny in Belgium ahead of Erwin Vervecken and future cyclo-cross world champion Niels Albert.
At the age of 24, Van den Broeck managed to finish in the top-10 on the general classification at the 2007 Eneco Tour with his new team, though he also won the bronze medal in the Belgian National Time Trial Championships. After completing his first Grand Tour in an anonymous 74th place at the 2007 Giro d'Italia, Van den Broeck finally showed that he had talent as a stage racer when he returned to Italy the following year and came seventh overall in the 2008 Giro d'Italia. Merely months later, both Riccardo Riccò and Emanuele Sella – respectively finishing 2nd and 6th in the 2008 Giro d'Italia – were sanctioned for doping.
Showing excellent form in the 2009 Tour de France, Van den Broeck climbed with the leaders through the Alps, although he had crashed heavily on the fourth stage team time trial, and lost more than seven minutes. His consistency resulted in 15th place in the final general classification ranking, which was later improved to 14th after the disqualification of 11th placed Mikel Astarloza for testing positive for EPO; he also placed 9th place in the competition for the polka dot jersey. As the performances came at the end of Cadel Evans' tenure at Silence–Lotto, the Belgian outfit decided that Van den Broeck was capable and reliable enough to become their general classification leader.
The following season he rode to an important and somewhat surprising 4th place (5th place before Alberto Contador's disqualification) in the 2010 Tour de France, becoming the first Belgian to finish in the top 5 since Claude Criquielion in 1986. After the Tour Van den Broeck won two criteriums: one in Peer and the other in his hometown of Herentals.
In the 2011 Tour de France, after an encouraging first week and while aiming for a general classification podium spot, Van den Broeck had to abandon the race along with another overall contender, Alexander Vinokourov, after a massive and spectacularly grim downhill crash which left him with a pneumothorax, broken ribs and a fractured shoulder. Not to be deterred, however, Van den Broeck recovered from his injuries in time to compete in the 2011 Vuelta a España, where he finished 8th overall.
In 2012, Van den Broeck finished 3rd in the Volta a Catalunya and then finished 5th in the Criterium du Dauphiné, showing good form for the forthcoming Tour de France. On Stage 7 of the Tour de France, a mountain top finish at La Planche des Belles Filles, Van den Broeck lost 1 minute 54 seconds to stage winner Chris Froome after getting a flat tyre just before the final climb. However Van den Broeck then showed his good form in the mountains to move him up to finishing 4th overall.
In 2013, Van den Broeck abandoned in the Tour de France after he crashed in stage 5.
In 2015, he finished twelfth at the Giro d'Italia. In June, he won the National Time Trial Championships for the first time in his career.
In September 2015, Team Katusha announced that Van Den Broeck would join them for 2016, after nine seasons with Lotto.
After one season with Team Katusha, LottoNL–Jumbo signed Van den Broeck on a two-year contract. In May, it was announced that Van den Broeck would retire at the end of the 2017 season.