With this organisation he was also present at a wide range of student activities, like the solidarity campaign with the workers of the shipyard Boelwerf in Temse, which was threatened with closure.

In 1994 Mertens became the president of the MLB, which then participated in the strike movement against proposed reforms by the Minister of Higher Education in the government of the French community, Michel Lebrun.

At the closing session on 2 March 2008, Mertens was elected as the successor of then party chairman Ludo Martens, who was having serious health problems.

Mertens declared in the Dutch language newspaper De Morgen that his party would leave behind its pedantic attitude and the big theories.

As chairman, he announced his intention to rethink the position of the party in accordance with the decisions taken at the congress, in the process turning his back on Maoism and Stalinism.

[5] This made him the fourth most popular politician in Antwerp, behind Bart De Wever, Patrick Janssens and Filip Dewinter.

[9] In the 2019 federal elections, Mertens was the lead candidate on the Workers' Party of Belgium list for the constituency of Antwerp and won a seat in Belgian Chamber of Representatives.