Born from a Serer father, Niokhor Diagne and a Manjack mother originating in Guinea-Bissau, Gnagna Anthony Preira, Galaye Mbaye Diagne was born in Gorée, Senegal, he was later adopted by a Christian family who baptised him Blaise. He studied in France before joining the French customs service in 1892. He served in Dahomey (modern day Benin), French Congo (now Republic of the Congo), Réunion, Madagascar, and French Guiana. In September 1899, while in Réunion, Diagne became a freemason, joining a lodge affiliated with the Grand Orient de France.
Diagne was elected to the Chamber of Deputies of France in 1914 as Senegal's representative. He was reelected several times, serving until his death in 1934. 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. In 1916 Diagne convinced the French parliament to approve a law (Loi "Blaise Diagne") granting full citizenship to all residents of the so-called Four Communes in Senegal: Dakar, Gorée, Saint-Louis, and Rufisque. This measure constituted a considerable element of the French colonial policy of a "civilizing mission" (mission civilisatrice). 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.
After the war, Diagne embarked on an administrative career in addition to his responsibilities as a parliamentary deputy. From October 1918 to January 1920 he served as Commissioner General of the Ministry of Colonies with supervision of military personnel from the colonies and workers from France's African possessions. He represented France in the International Labor Office, the secretariat of the International Labour Organization, in 1930. From January 1931 to February 1932 he was Under-Secretary of State for the Colonies, a junior level cabinet position. From 1920 to 1934 he served as mayor of Dakar.
He died in Cambo-les-Bains in 1934.
He was a pioneer of black African electoral politics and an advocate of equal rights for all, regardless of race. He encouraged African accommodation of French rule and the adoption of French cultural and social norms. Though he was ahead of his time in 1914, by the later years of his life, African colonial politics had passed him by. He continued to advocate an African role in France while most Western-educated African elites embraced African nationalism and worked for eventual independence from the colonial powers.
It is alleged that he was not buried in the Muslim cemetery of Soumbedioune in Dakar because of his freemasonry. However, a large boulevard (Avenue Blaise Diagne) and a high school (Lycée Blaise Diagne) in Dakar were named in his honor, as well as Senegal's new international airport, Blaise Diagne Airport in Ndiass, 52 kilometers (32 mi) outside of Dakar.
His son Raoul was the first black to play professional soccer in France and had great success playing for Racing Club de France in the late 1930s, winning the French title in 1936 and the French cup in 1936, 1939, and 1940.
His like-named grandson was born in Paris in 1954 to his son Adolphe (1907-1985, a French medical officer). The younger Blaise became mayor of the French village of Lourmarin (1,002 inhabitants in 2010) in the Lubéron mountains of Provence in 2001 and was reelected in 2008. According to him the memory of his grandfather was scarcely mentioned within the family ("Mais mes parents ont toujours été très discrets sur cette histoire familiale"). His mother and grandmother were both French "white" women. When interviewed in 2005, the younger Blaise said he had not travelled to Senegal since 1960 and thought he "has nothing to bring there".