He is best known for his unsuccessful attempt with Camille Huysmans and others to establish an unrecognised Belgian government in London in 1940 during World War II.

He held ministerial appointments in the coalition governments of Paul Van Zeeland and Hubert Pierlot, latterly during the German invasion of Belgium in May 1940.

Because of the international nature of Ernest's work, Marcel-Henri spent much of his youth overseas in Cairo, Egypt (1905–15) and Paris, France (1915–19).

He practised briefly as a lawyer at the Court of Appeal in Brussels, and subsequently published a study on Belgian company law.

[2] He quickly gained a reputation as one of leading figures in the party's "Young Turks" faction who emphasized the need for social reform besides the traditional liberal economic agenda of laissez-faire.

Léon Degrelle's Rexist Party launched a number of visceral public attacks on Jaspar as a former director of the defrauded Constructa company as part of a wider denunciation of supposed "politico-financial scandals" associated with the government of Van Zeeland.

[2] Following the German invasion of Belgium (10–28 May 1940), the Belgian government was forced to withdraw from Brussels into France, eventually to Bordeaux.

[5] In London, Jaspar was soon joined by other left-wing politicians, many of them radicals, such as Camille Huysmans, Max Buset, and Isabelle Blume.

He held the post until his retirement in 1968, a term that coincided with the presidency of Charles de Gaulle and the Algerian War.

[2] During his career, Jaspar published numerous books and newspaper articles on political issues, history, and law.