His other notable works include The Last Day (2004), Paris, je t'aime (2006), Jacquou le Croquant (2007), The Princess of Montpensier (2010), To the Ends of the World (2018), Sibyl (2019), and Twice Upon a Time (2019).

[15][20][18] He dreamed of becoming a film director and attended the University of Saint-Denis in Paris, where he studied cinema for two years until he quit to focus on his acting career.

"[22] He made his acting debut with a small part in one episode of Une femme en blanc, a 1997 miniseries for French television starring Sandrine Bonnaire.

[24] In 2001, Ulliel made his feature film debut with a minor role in Christophe Gans's Brotherhood of the Wolf, alongside Vincent Cassel and Monica Bellucci.

[24][25] For his breakthrough performance in the film,[29] Ulliel was nominated for a César Award for Most Promising Actor,[26] and won the Étoiles d'Or for Best Male Newcomer.

[30] Ulliel had his first English-speaking role in Peter Greenaway's 2004 film The Tulse Luper Suitcases, Part 2: Vaux to the Sea, where he played a French man named Leon.

[31] He also starred as Joseph, the son of Isabelle Huppert's character in Rithy Panh's drama The Sea Wall, based on the 1950 novel of the same name by Marguerite Duras.

[31] In 2010, he portrayed Henry I, Duke of Guise in Bertrand Tavernier's romance film The Princess of Montpensier, based on the 1662 novel of the same name by Madame de La Fayette.

[43] In 2015, Ulliel starred as Tomas in an adaptation of Lars Norén's play Démons, directed by Marcial Di Fonzo Bo at the Théâtre du Rond-Point in Paris.

[38] In 2016, he appeared in two films: the lead role in Xavier Dolan's It's Only the End of the World as Louis, a playwright who returns home after twelve years to tell his family that he's dying, in which he starred opposite Marion Cotillard, Vincent Cassel, Léa Seydoux and Nathalie Baye;[44] and in Stéphanie Di Giusto's The Dancer, an unconventional biopic of American dancer Loie Fuller where Ulliel portrayed Count Louis d'Orsay.

[45] In 2017, he was nominated for the Jury Prize for Best Actor at the Riviera International Film Festival for his role in Xavier Dolan's It's Only the End of the World alongside co-star Vincent Cassel.

[31] In 2019, he portrayed actor Igor Maleski in Justine Triet's comedy-drama film Sibyl;[31] and played the lead role in the Arte/Netflix's time-travel miniseries Twice Upon a Time, directed by Guillaume Nicloux, where Ulliel starred as Vincent Dauda, a man who gets delivered by mistake a mysterious wooden box that allows him to travel back in time to save his relationship with his ex-girlfriend, portrayed by Freya Mavor.

[63] In July 2021,[64][65] Ulliel joined the cast of the Disney+ streaming superhero miniseries Moon Knight,[66] set in the Marvel Cinematic Universe and released on 30 March 2022.

[67] He portrayed the villain Anton Mogart / Midnight Man opposite Oscar Isaac, Ethan Hawke and May Calamawy, guest starring in the episode "The Friendly Type".

[74] In December 2021,[75] Ulliel started shooting Of Money and Blood, a Canal Plus crime thriller miniseries about the carbon tax fraud where he would play a character inspired by French businessman Arnaud Mimran.

Shot in Bangkok and London, it featured Ulliel crossing the city looking for a woman while David Bowie's "Starman" plays in the background.

[106] On season 4, episode 5 of Call My Agent!, Sigourney Weaver (playing herself) wants Ulliel to be cast as the male lead in her upcoming film.

"[133] In 2019, he signed the petition "Résister et créer" ("Resist and Create"), created by the movement On Est Prêt along with Cyril Dion to challenge the world of cinema at a time of mobilizations for the climate,[134][135] and also narrated the campaign "Nos objets ont plein d’avenirs" ("Our objects have plenty of future"), designed by DDB Paris for Ademe and the Ministry of Ecological and Solidarity Transition, which highlights all the actions to be taken to extend the life of objects and help preserve the planet's resources and reduce the production of waste.

[138] Ulliel was airlifted by helicopter to the trauma unit of the Centre Hospitalier Universitaire Grenoble Alpes in nearby La Tronche,[139][140] where he died the following day around 4:00 p.m. at the age of 37.

[138] La Rosière's director, Jean Regaldo, told France's BFM TV that Ulliel was not wearing a helmet when rescuers arrived.

[150] The resort, La Rosière, also paid tribute to Ulliel by stopping the ski lifts for one minute at 3:58 pm, the time of his accident on 18 January.

[152] Among those who paid tribute to Ulliel following his death were France's president Emmanuel Macron,[153] who described Ulliel as "an icon of French elegance who dazzled the cameras",[154] France's Prime Minister Jean Castex,[24] and France's Culture Minister Roselyne Bachelot,[24] as well as peers and collaborators such as Xavier Dolan,[24] Martin Scorsese,[24] Marion Cotillard,[155] Isabelle Huppert,[156] Juliette Binoche,[157] Chanel,[99] Disney,[24] Cannes Film Festival director Thierry Fremaux, Cannes Film Festival president Pierre Lescure,[157] Jean Dujardin,[24] Peter Webber,[157] Pierre Niney,[24] Louis Garrel,[156] André Téchiné,[158] and Bertrand Bonello.

"[162] Dolan's speech was followed by the ceremony's "In Memoriam" segment, which also began with a tribute to Ulliel featuring scenes from the films A Very Long Engagement and Saint Laurent.

[164] Despite not being mentioned during the Oscars' In Memoriam segment on 27 March 2022,[165] Ulliel was included in the online memorial from the official website of the Academy of Motion Picture Arts and Sciences among other artists who had died recently.

[166] On 13 April 2022, that week's episode of the Marvel Studios miniseries Moon Knight, which featured Ulliel guest starring in the role of Anton Mogart, was dedicated to his memory with a tribute in the end credits.

[168] Ulliel's last partner, Gaëlle Pietri, wrote a book paying tribute to him titled Le Temps de te dire adieu ("Time to Say Goodbye"), published on 26 April 2023.

[169] Jean-Marc Peillex, the mayor of Saint-Gervais-les-Bains in Haute-Savoie, stated that they must impose helmet-wearing to reduce the risk of serious accidents and also reexamine on a case-by-case basis the dangerousness of different pistes and potentially reclassify their colors.

[169] When asked if helmets should be made compulsory for skiing in France – like they are in other European countries – during a press conference on 20 January 2022, government spokesperson Gabriel Attal said that it was "not the moment to begin a debate around the subject.

"[169] It was reported that 97% of children under the age 12 do wear a helmet in France, but the National Mountain Safety Observation System noted that this percentage decreases as people get older.

Pietri said: "Gaspard was a diver, sensitive to climate issues and concerned about the protection of underwater biodiversity, he would have liked the momentum generated by his death to go to the defense of these causes that were close to his heart.