Poulter played various acting roles before landing the role of Lee Carter in the 2007 movie Son of Rambow, which was released to positive reviews, and praise for the performances of Poulter and his co-star Bill Milner. He also performed with other young comedic actors in School of Comedy, which aired its pilot on Channel 4's Comedy Lab on 21 August 2008. School of Comedy was then commissioned for a full series by Channel 4, which began airing on 2 October 2009. The program finished after a second series.

In 2009, he was selected to play the role of Eustace Scrubb in the film The Chronicles of Narnia: The Voyage of the Dawn Treader (filmed in Queensland, Australia), and was accompanied by some members of his family. The movie was first screened on 10 December 2010. The film opened to mixed reviews, but Poulter's performance was well received.

In 2010, he appeared in the BBC Three pilot The Fades, a 60-minute supernatural thriller written by Skins writer, Jack Thorne. The pilot has been commissioned to be written as six-part series with almost entirely a new cast.

Poulter began filming a small British independent film called Wild Bill, directed by Dexter Fletcher, at the end of 2010. It centres around Bill Hayward, played by Charlie Creed-Miles, who, on parole after spending eight years in prison, finds his two sons, Dean (Poulter) and Jimmy (Sammy Williams), living alone abandoned by their mother. With the attention of social services now focused on the boys, Bill struggles to play good dad while keeping out of jail as Jimmy gets in trouble with some dangerous acquaintances of Bill's past. The film was released on 23 March 2012 to extremely positive reviews, with praise for Poulter's performance.

In 2011, Poulter appeared with the popular British blogger and his School of Comedy co-star Jack Harries on his YouTube channel by the name of JacksGap in a video called "Jack and Will".

In 2013, he played Kenny in We're the Millers, starring Jennifer Aniston and Jason Sudeikis. While the film opened to mixed reviews, the performances of the cast were well-received, especially Poulter. He also appeared as a caretaker in the music video for Rizzle Kicks' song "Skip to the Good Bit".

In 2014, he played Fordy in the crime film Plastic, directed by Julian Gilbey and starring Ed Speleers, Alfie Allen, Sebastian De Souza and Emma Rigby. The film was critically panned on release. The same year, he played Gally in the film adaptation, The Maze Runner, alongside Dylan O'Brien and Kaya Scodelario. The film was a critical and commercial success, with the performances of the cast being praised.

In 2015, Poulter starred as Shane in the Irish indie film Glassland, directed by Gerard Barrett and co-starring Jack Reynor and Toni Collette. The film was a critical success, with many reviewers praising Poulter's performance in particular as being his most diverse role to date. In an interview with BBC Radio 1, Poulter stated the film was "the proudest I've been to be a part of a movie".

In 2014, Poulter won the BAFTA Rising Star Award, voted for by the public. Other actors nominated for the award were Lupita Nyong'o, George MacKay, Léa Seydoux and Dane DeHaan. The same year, he also won the MTV Movie Award for Best Breakthrough Performance and the MTV Movie Award for Best Kiss (shared with his co-stars Jennifer Aniston and Emma Roberts) for his performance in We're the Millers.

In 2014, Poulter was chosen as one of 23 upcoming actors to feature in July's issue of Vanity Fair, with all actors being named "Hollywood's Next Wave". Other actors featured included Dylan O'Brien (Poulter's co-star in The Maze Runner), Jack Reynor (Poulter's co-star in Glassland), and Tye Sheridan (Poulter's co-star in The Yellow Birds).

Poulter played Jim Bridger in the revenge-thriller The Revenant, directed by Alejandro González Iñárritu, and starring Leonardo DiCaprio and Tom Hardy. The film centers on an 1820s frontiersman on a path of vengeance against those who left him for dead after a bear mauling. In 2017, he played a police officer in the film Detroit, about the 1967 Detroit riots.

Poulter was reported to have been cast in the role of Pennywise the Clown in the 2017 remake of the Stephen King miniseries It. However, it was announced in June 2016 that Bill Skarsgård was cast instead, since Poulter was forced to drop out due to both scheduling conflicts (the film was set to shoot at the same time as Poulter was set to begin work on Detroit), as well as the departure of its initial director, Cary Fukunaga.

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