Hermant was born in Paris, the son of an architect.
He received a degree from the École Normale Supérieure in 1880, and published his first volume of verse in 1883, The Contempt. After several youthful novels, he moved to ironic analysis of the popular mores of the Belle Époque and achieved popular success. His first semi-autobiographical novel, Monsieur Rabosson of 1884, established his reputation as a satirical social observer. Its follow-up Le Cavalier Miserey of 1887, dealt with the issue of homosexuals in the military.
Between 1901 and 1937 Hermant embarked on a series of 20 linked novels with the general title Memoirs to Serve for a History of Society, but his contributions to literature included many popular plays, drama criticism for Le Figaro and Gil Blas, and a series of grammarian articles for Le Temps under the name "Lancelot" defending the purity of the French language.
By 1899 Hermant was well-connected in society; for instance he was the guest of Anna de Noailles at Évian-les-Bains, where he became friends with Marcel Proust. After a number of tries Hermant was elected to the French Academy on 30 June 1927.
During World War II Hermant's contributions to Jean Luchaire's pro-Nazi evening daily Les Nouveaux Temps, beginning in 1940, his open support of the Vichy regime, and his criticisms of the French Army, marked him as a collaborator. Over 80 years of age, he was sentenced to life in prison on 15 December 1945. Hermant achieved the negative distinction of being one of the four "immortals" removed from the Académie Française after World War II for collaboration with Germany. Hermant and Abel Bonnard were expelled outright, in disgrace; Charles Maurras of Action Française and Marshall Philippe Pétain had their seats declared vacant and were not replaced until their deaths.
Pardoned and released in 1948, Abel Hermant tried to justify its conduct during the Occupation in his Thirteenth Notebook. He died shortly thereafter.