Born in Gorée to a Senegalese Lebu father—Niokhor Diagne—a cook and sailor, and a Manjack mother of Guinea-Bissau origin—Gnagna Anthony Preira.

Diagne was adopted as a child by the Crespin family who were of mixed race origin from Gorée and Saint-Louis, and Christians.

In September 1899, while in Réunion, Diagne became a freemason, joining a lodge affiliated with the Grand Orient de France.

From 1914 to 1917 he caucused with the Marxist-socialist Section française de l'Internationale ouvrière, forerunner of the French Socialist Party, before affiliating with the Independents led by Georges Mandel.

In 1914 after recently becoming the newly elected deputy of Senegal, Blaise Diagne was critical in the government intervention in an outbreak of plague which struck Dakar.

As part of Diagne's deal with the French to grant citizenship, he helped them organize military recruitment in Senegal.

He was a leading recruiter for the French army during World War I, when thousands of black West Africans fought on the Western Front for France.

Du Bois faulted him harshly for a perceived lack of commitment to African interests "...Diagne is a Frenchman who is accidentally black.