Charles Peckham Day was born in New York City, NY, and raised in Middletown, Rhode Island. His parents are both music teachers - his mother, Mary (Peckham), is a piano teacher, and his father, Dr. Thomas Charles Day, is a professor of Undergraduate Performing Arts at Salve Regina University in Newport, Rhode Island. Charlie plays both piano and guitar. He has Italian (from his paternal grandfather), Irish, and English ancestry.
He attended Merrimack College in Massachusetts, majoring in art history. After graduating, he began acting at the Williamstown Theatre Festival and in small television roles. He supplemented his income by waiting tables and answering phones for a telethon selling a Motown anthology. He got his first agent after performing a funny blues song he'd written at a cabaret night.
His big break came in 2005 after he pitched a home video he shot with his friends Rob McElhenney and Glenn Howerton. This eventually became the hit series It's Always Sunny in Philadelphia (2005). As well as starring in the show, Charlie is also an Executive Producer and one of the writers. His breakthrough film role was Horrible Bosses (2011).