Day is the second of five children born to Irish immigrants Patrick McNulty and Mary (née Grady) McNulty. He graduates from Cathedral Preparatory School and Seminary in New York City, and attends Manhattan College in the Bronx, where he sings in the glee club.
Mary Livingstone, wife of comedian Jack Benny, brings Day to Benny’s attention after hearing him on the radio during a visit to New York. She takes a recording of Day’s singing to Benny, who then goes to New York to audition Day. The audition results in Day’s role on the Benny program.
Day appears for the first time on Benny’s radio show on October 8, 1939, taking the place of another famed tenor, Kenny Baker. He remains associated with Benny’s radio and television programs until Benny’s death in 1974. He is introduced as a young, naive boy singer, a character he keeps through his whole career.
From 1944 through 1946 Day serves in the United States Navy as a Lieutenant. While in service he is temporarily replaced on the Benny radio program by fellow tenor Larry Stevens. On his return to civilian life, he continues to work with Benny while also starring on his own NBC show, A Day in the Life of Dennis Day (1946–1951). His last radio series is a comedy/variety show that airs on NBC’s Sunday afternoon schedule during the 1954–55 season.
An attempt is made to adapt A Day in the Life Of Dennis Day as an NBC filmed series, produced by Jerry Fairbanks for Dennis’ sponsor, Colgate-Palmolive, featuring the original radio cast, but gets no farther than an unaired 1949 pilot episode. Eventually, his own TV series, The Dennis Day Show, is first telecast on NBC on February 8, 1952, and then in the 1953–1954 season. Between 1952 and 1978, he makes numerous TV appearances as a singer, actor and voice for animation, such as the Walt Disney feature Johnny Appleseed, handling multiple characters. His last televised work with Benny is in 1970, when they both appear in a public service announcement together to promote savings and loans.
Although his career is mainly radio and TV-based, Day also appears in a few films. These include Buck Benny Rides Again (1940) opposite Jack Benny, Sleepy Lagoon (1943), Music in Manhattan (1944), I’ll Get By (1950), Golden Girl (1951), The Girl Next Door (1953), and Won Ton Ton, the Dog Who Saved Hollywood (1976) as a singing telegraph man. For the soundtrack of My Wild Irish Rose (1947), a biopic about Chauncey Olcott, Day provides the singing voice to the acting of Dennis Morgan.
Dennis Day dies on June 22, 1988, of amyotrophic lateral sclerosis (ALS), more commonly known as Lou Gehrig’s disease, in Los Angeles, California. His star on the Hollywood Walk of Fame is at 6646 Hollywood Boulevard. He is interred in Holy Cross Cemetery in Culver City.