Born in Paris, Séjan was one of the best organists of his time. He was co-titular of the organ of Notre-Dame de Paris and many other Parisian churches. He was appointed to the Chapelle Royale in 1789. He lost his charges during the French Revolution, but later recovered some of them.
- Honegger, Marc (1979). Dictionnaire de la musique; Tome 2, Les Hommes et leurs œuvres. L-Z (in French). Bordas. p. 1030. ISBN 2-04-010726-6.
- Before 1789, at the request of Étienne-Nicolas Méhul, he gave organ lessons to the young organist Georges Scheyermann. — cf: François-Joseph Fétis, Biographie universelle des musiciens et bibliographie générale de la Musique, Paris : Firmin Didot, 1867, 2nd éd., vol.7, p. 455 .
- "Nicolas Séjan (1745 - 1819)". Find A Grave Memorial. Retrieved 2017-10-13.