At the age of 17, Alain Lenglet decided to dedicate himself to the theatre. He enrolled at the Conservatoire national d’art dramatique de Paris (Paris Conservatory) where he studied under Pierre Vial, Marcel Bluwal and Pierre Debauche. Upon graduation, he worked with directors of great renown, such as: Jacques Rosner in Shakespeare’s Macbeth; Jean-Pierre Miquel in Louis Calaferte's Tu as bien fait de venir, Paul; Harold Pinter’s The Collection and Albert Camus’ Les Justes; Daniel Mesguich in Shakespeare’s King Lear; Pierre Debauche in Shakespeare’s Othello; and Augusto Boal in The Incredible and Sad Tale of Innocent Eréndira and her Soulless Grandmother by Gabriel-Garcia Marquez. In 1992, he created Jean-Pierre Klein’s Contes d’avant l’oubli after I. B. Singer, which was directed by Jean-Luc Porraz. Alain Lenglet was part of both Laurent Terzieff’s Company and Pierre Pradinas’s Chapeau Rouge.

He was engaged by the Comédie-Française on the 1st of December 1993, and became the 502nd sociétaire on the 1st of January 2000.
Within the Company, he has distinguished himself in performing Molière’s emblematic characters such as Clitandre in Les Femmes savantes, which was directed by Simon Eine; Ariste in L’École des maris and Sganarelle in Le Cocu imaginaire by Thierry Hancisse; Monsieur de Sotenville in George Dandin first by Catherine Hiegel and later by Hervé Pierre; and Béralde in Le Malade imaginaire staged by Claude Stratz. Concurrently, he explored great texts from the repertoire such as Corneille’s Clitandre ou l’Innocence délivrée staged by Muriel Mayette-Holtz; Shakespeare’s Taming of the Shrew staged by Oskaras Koršunovas; Corneille’s L’Illusion comique by Galin Stoev; Eduardo de Filippo’s La Grande magie and Shakespeare’s Hamlet under the direction of Dan Jemmett. Alain Lenglet rejoined Jean-Pierre Miquel for Marivaux’s La Double Inconstance; created Roland Topor’s L’Hiver sous la table directed by Claude Confortés; performed in Chekhov’s Cherry Orchard directed by Alain Françon; in Gogol’s Marriage; and in Marcel Aymé’s La Tête des Autres by Lilo Baur. Éric Ruf, who directed him in Robert Garnier’s Le Laboratoire des formes in 2004, entrusted him with the role of Osmin in Racine’s Bajazet in 2017. The same year, Isabelle Nanty cast him in L’Hôtel du Libre-Échange by George Feydeau.

In 2008, Alain Lenglet co-directed with Marc Fayet La seule certitude que j’ai, c’est d’être dans le doute, after Pierre Desproges. For the young audience, he created Fabrice Melquiot’s Bouli Miro directed by Christian Gonon and, in 2016, played Marcel Aymé’s Le Cerf et le Chien by Véronique Vella.

Beside Mathieu Amalric’s adaptation of Corneille’s L’Illusion Comique and Vincent Macaigne’s adaptation of Molière’s Dom Juan, Alain Lenglet’s filmography includes regular collaborations with Robert Guédiguian: Ki lo sa?; Dieu vomit les tièdes; À la vie, à la mort; À l’attaque; La ville est tranquille; L’Armée du crime and Une histoire de fou.

Alain Lenglet holds the honors of Chevalier of the Order of Arts and Letters. (Chevalier de l’ordre des Arts et des Lettres).

During the 2017/2018 season, Alain Lenglet interpreted D in Poussière by and directed by Lars Norén, Montaigu in the rerun of Shakespeare’s Romeo and Juliet by Éric Ruf, Ernest in the rerun of Feydeau’s Free Exchange Hotel directed by Isabelle Nanty, the Father in the rerun of Marcel Aymé’s Le Cerf et le chien directed by Véronique Vella and a Profesor in Wedekind’s Spring Awakening staged by Clément Hervieu-Léger.

During the 2018/2019 season, he will be playing in Le Voyage de G. Mastorna after Federico Fellini by Marie Rémond and and in the rerun of Molière's Misanthrope, in the rerun of Feydeau’s Free Exchange Hotel and in the rerun of Hugo's Lucrèce Borgia.