Jennifer Decker’s drama teacher in Secondary school was author and stage director Pierre Notte. He introduced her to Irina Brook who offered her the role of Juliet in her retelling of the Shakespearean classic Juliette et Roméo, staged at the Théâtre national de Chaillot in 2001, before embarking on a long tour.
Since 2003, she has appeared in a number of television films, such as Les Amants naufragés directed by Jean-Christophe Delpias; Les Amants du Flore by Ilan Duran Cohen_;_ and Mange by Julia Ducournau and Virgile Bramly. For the cinema, Jennifer Decker was directed by Tony Bill in Flyboys and by Christoph Schaub in Jeune Homme, both in 2006. The following year, James Huth cast her for Hellphone. In 2009, she played in two feature films, Oskar Roehler's Lulu und Jimi and Bank Error in Your Favour (Erreur de la banque en votre faveur) by Michel Munz and Gérard Bitton. In 2015, she performed in Christian Vincent’s movie Courted (L’Hermine).
Jennifer Decker returned to the theatre in 2008, when she participated in the creation of Louis Jouvet – Romain Gary 1946-1951 at the Théâtre Vidy-Lausanne, under the direction of Gabriel Garran. The following year, her old teacher Pierre Notte, directed her in his own work Les Couteaux dans le dos, les ailes dans la gueule at the Théâtre Les Déchargeurs in Paris. Later, alone on stage she performed Manon Heugel’s work Novembre, déjà directed by the author.

Since the 1st of September 2011, Jennifer Decker has been pensionnaire of the Comédie-Française, where she made her debut as Mariane in Molière’s The Miser (L’Avare) by Catherine Hiegel. Since then, she most notably collaborated with Lilo Baur in Gogol’s Marriage (Le Mariage), Marcel Aymé's La Tête des autres and Federico García Lorca's The House of Bernarda Alba (La Maison de Bernarda Alba); Jean-Pierre Vincent in his staging of Molière's Dom Juan; Christian Benedetti in Lampedusa Beach by Lina Prosa; Giorgio Barberio Corsetti in his staging of Labiche’s work Un chapeau de paille d’Italie; Marc Paquien in Antigone by Jean Anouilh; Clément Hervieu-Léger in Molière's Misanthrope; Anne Kessler in Marivaux’s La Double Inconstance; and Alain Françon in The Sea by Edward Bond. Jennifer Decker has excelled in as Doña Sol in Victor Hugo’s Hernani by Nicolas Lormeau; Aricie in Michael Marmarinos’s version of Phèdre by Racine; Ophelia in Shakespeare’s Hamlet directed by Dan Jemmett; Cléanthis in Marivaux’s L’Île des esclaves by Benjamin Jungers, Iris in the play Une vie written and directed by Pascal Rambert; and as Olga in Les Damnés, a play by Ivo van Hove after the screeplay by Luchino Visconti, Nicola Badalucco and Enrico Medioli.

In 2018, Jennifer Decker played the title role in Seneca’s Phèdre by Louise Vignaud and was cast by David Lescot in his own play Les Ondes magnétiques.