Charlie Day made his debut in the 2000 American made-for-television comedy-drama film ‘Mary and Rhoda’ directed by Barnet Kellman. The film received mostly negative reviews from critics. That same year, he also appeared in the Episode: "Three Guys, a Girl and a Conversation Nook" of the American sitcom ‘Madigan Men.’ The following year, he starred as ‘Joe Boner’ in the horror mystery film ‘Campfire Stories.’ The film failed to impress the critics. He next appeared in television series like ‘Law & Order’ (2001), ‘Third Watch’ (2001-2004), Luis’ (2003), and ‘Reno 911!’ (2004). In 2005, he joined the cast of the American sitcom ‘It's Always Sunny in Philadelphia’ created by Rob McElhenney. The show’s fifteenth season premiered in December 2021. In 2010, he starred as ‘Dan Grant’ in the American romantic comedy film ‘Going the Distance’ directed by Nanette Burstein. The film received a mixed response from critics and grossed over $42.1 million on a budget of $32 million. In 2011, he starred as ‘Dale Arbus’ in the American black comedy film ‘Horrible Bosses’ directed by Seth Gordon. The film received mixed to positive reviews from critics and was a major financial success, grossing over $209.6 million on a budget of $35–37 million.
In 2014, he starred as ‘Dr. Newton Geiszler’ in the American science fiction monster film ‘Pacific Rim’ directed by Guillermo del Toro. The film received mostly positive reviews and was a commercial success. The following year, he reprised his role as ‘Dale Arbus’ in the ‘Horrible Bosses 2,’ the sequel to the 2011 film ‘Horrible Bosses.’ The film opened to a mixed response from critics and was a financial success. Day also received 3 ‘Teen Choice Award’ nominations for his performance in the film. He was next seen in films like ‘Vacation’ (2015), ‘The Hollars’ (2016), ‘Fist Fight’ (2017), and ‘I Love You, Daddy.’ In 2018, he returned to play ‘Dr. Newton Geiszler’ in the American science fiction monster film ‘Pacific Rim: Uprising’ directed by Steven S. DeKnight. The film received mixed reviews from critics and was a box office disappointment. That same year, he also appeared in the American dystopian tech-noir thriller film ‘Hotel Artemis’ directed by Drew Pearce. Despite opening to mixed reviews, the film tanked at the box office.
In 2019, he voiced the character ‘Benny’ in the computer-animated adventure comedy film ‘The Lego Movie 2: The Second Part’ directed by Mike Mitchell. Even though the film received positive reviews from critics, it turned out to be a box office disappointment. Day was last seen in the 2022 American romantic comedy film ‘I Want You Back’ directed by Jason Orley. The film earned praise from critics, He is all set to appear in the upcoming comedy film ‘El Tonto.’