Day was born and raised in New York City, the son of Irish immigrants. His father was a stationary engineer. Day graduated from St. Patrick's Cathedral High School, and attended Manhattan College, where he sang in the glee club.
Besides singing, Dennis Day was an excellent mimic. He did many imitations on the Benny program of various noted celebrities of the era, such as Ronald Colman, Jimmy Durante, and Jimmy Stewart.
From 1944 through 1946, he served in the US Navy as a Lieutenant. On his return to civilian life, he continued to work with Benny while also starring his own show, A Day in the Life of Dennis Day (1946-1952). Day's having two programs in comparison to Benny's one was the subject of numerous jokes and gags on Benny's show, usually revolving around Day rubbing Benny's, and sometimes other cast members and guest stars' noses in that fact.
In 1948, Day married Peggy Almquist; the marriage lasted until his death in 1988. The couple had ten children. One of his brothers was wed to actress/singer Ann Blyth.
Dennis Day died of Lou Gehrig's disease at age 72 in Los Angeles, California. His star on the Hollywood Walk of Fame is at 6646 Hollywood Boulevard. He is interred at Holy Cross Cemetery.