He is best known for playing Charlie Kelly on the FX dark comedy It's Always Sunny in Philadelphia (2005–present), which he stars in with Rob McElhenney, Kaitlin Olson, Glenn Howerton and Danny DeVito, and of which he is also a writer and an executive producer.

[2] He subsequently co-created the Fox sitcom The Cool Kids (2018–2019) with Paul Fruchbom and the Apple TV+ comedy Mythic Quest (2020–present) with McElhenney and Megan Ganz, and continues to executive-produce the latter.

[5] His paternal grandfather changed the family name from Del Giorno to Day[6][7] to assimilate during WWII;[7] he died in a military training accident when his son Thomas was only four.

[9] In May 2014, he gave the commencement speech for Merrimack's graduating class and received an honorary Ph.D.[10] While still in college, Day was active in the training programs at the Williamstown Theatre Festival every summer from 1997, where he was a contemporary of actors such as Jimmi Simpson, David Hornsby, Kathryn Hahn, Justin Long and Sterling K. Brown.

[11] After graduating, Day worked on small television roles, advertisements, and voiceovers for the Independent Film Channel,[12] and supplemented his income by waiting tables and answering phones for a telethon.

In the early years of his career, Day often made comedy sketches and absurd short films in his spare time with Jimmi Simpson, with whom he was living in New York City, and several friends including David Hornsby, Nate Mooney, Logan Marshall-Green and other actors, many of whom they had met through the Williamstown Theatre Festival.

[14] These home videos served as the inspiration for several scripted short films he later developed with Rob McElhenney and Glenn Howerton in 2003, once he had moved out to Los Angeles.

Among these home movies were two scenes about three struggling self-involved actors in Los Angeles getting into awkward and darkly comedic situations between auditions and jobs,[15] which went on to form the basis of the pilot episode of the comedy series that would go on to be known as It's Always Sunny in Philadelphia.

In 2017, he co-created the Fox sitcom The Cool Kids (2018–2019), starring Vicki Lawrence, Martin Mull, David Alan Grier and Leslie Jordan and set in a retirement community.

On August 9, 2019, Mythic Quest (2020–present), a new half-hour comedy series co-created by Day, McElhenney and Megan Ganz, who is also an executive producer on Sunny, was announced as one of the original productions for the then-upcoming streaming service, Apple TV+.

[citation needed] Day had a significant role in the Guillermo del Toro science fiction kaiju film Pacific Rim (2013), in which he played biologist Dr. Newton "Newt" Geiszler, who is the focus of the secondary comedic plot with Burn Gorman and Ron Perlman.

and featured himself, Chloe Grace Moretz, John Malkovich, Rose Byrne, Pamela Adlon, Edie Falco and Helen Hunt.

[34] In addition to Pacific Rim Uprising, in 2018, Day was in Drew Pearce's film Hotel Artemis, with Jodie Foster, Sterling K Brown, Sofia Boutella, Jeff Goldblum, Brian Tyree Henry, Jenny Slate, Zachary Quinto and Dave Bautista, where he played Acapulco the arms dealer.

[35] Most recently, Day played his first leading role in a romantic comedy as Peter on I Want You Back with Jenny Slate for Amazon Studios released on February 11, 2022.

[40] Besides writing and producing, Day co-stars as a silent man from a psychiatric hospital who accidentally finds his way into celebrity with the help of an enterprising publicist (Jeong) before they lose it all.