Nicknamed "the boss" (le patron), Vandervelde was a leading figure in the Belgian Labour Party (POB–BWP) and in international socialism.

[1] However, he soon became interested in emerging socialist ideas and, in 1885, joined the small Workers' League of Ixelles (Ligue Ouvrière d'Ixelles).

Vandervelde was active in Belgian Freemasonry and was a member of the Lodge Les Amis philanthropes du Grand Orient de Belgique, in Brussels.

[2] Following the extension of universal male suffrage in 1893, Vandervelde proposed a manifesto for the POB, known as the Charter of Quaregnon which would form the basis for Belgian socialist politics until the 1970s.

He was a staunch opponent of Leopold II and the absolute power he enjoyed in the Congo during the 1890s and wrote numerous articles against capitalist colonialism.

During the Spanish Civil War, Vandervelde's desire to intervene to halt the growing threat of fascism was challenged by Henri de Man and Paul-Henri Spaak.