In 1939 Gene McNulty, as Day was then known, sang on network radio with bandleader Larry Clinton. The Clinton broadcasts were aimed at the collegiate audience, and were often broadcast from a college campus. The 23-year-old McNulty won an audience poll as a favorite vocalist.

Day appeared for the first time on Jack Benny’s radio show on October 8, 1939, taking the place of another tenor, Kenny Baker. He remained associated with Benny’s radio and television programs until Benny’s death in 1974. He was introduced (with actress Verna Felton playing his overbearing mother) as a young (nineteen-year-old), naive boy singer – a character he kept through his whole career.

From 1944 through 1946 he served in the United States Navy as a Lieutenant. While in service he was temporarily replaced on the Benny radio program by fellow tenor Larry Stevens. On his return to civilian life, he continued to work with Benny while also starring on his own NBC show, A Day in the Life of Dennis Day (1946–1951). On Benny’s show, Day’s having two programs in comparison to Benny’s one was the subject of numerous jokes and gags, usually revolving around Day rubbing Benny’s, and sometimes other cast members’ and guest stars’ noses in that fact (e.g., “Dennis, why do you have two horns on your bicycle?” “Why shouldn’t I? I’ve got two shows!”). His last radio series was a comedy and variety show that aired on NBC’s Sunday afternoon schedule during the 1954–55 season.

In 1948, Day married Peggy Almquist; the marriage lasted until his death in 1988. The couple had ten children. His brother Jim McNulty, two years younger, was married to actress and singer Ann Blyth.

An attempt was made to adapt A Day in the Life Of Dennis Day as an NBC filmed series (Sam Berman’s caricature of Dennis was used in the opening and closing titles), produced by Jerry Fairbanks for Dennis’ sponsor, Colgate-Palmolive, featuring the original radio cast, but got no farther than an unaired 1949 pilot episode. In late 1950, a sample kinescope was produced by Colgate and their ad agency showcasing Dennis as host of a projected “live” comedy/variety series (The Dennis Day Show) for CBS, but that, too, went unsold. He continued to appear as a regular cast member when The Jack Benny Program became a TV series, staying with the show until it ended in 1965.

Eventually, his own TV series, The Dennis Day Show (aka The RCA Victor Show), was first telecast on NBC on February 8, 1952, and then in the 1953–1954 season. Between 1952 and 1978, he made numerous TV appearances as a singer and actor (such as NBC’s The Gisele MacKenzie Show and ABC’s The Bing Crosby Show and Alfred Hitchcock Presents) and voice for animation.

A Republican, Day was supportive of Dwight Eisenhower’s campaign during the 1952 presidential election and Barry Goldwater in the 1964 United States presidential election.

His last televised work with Benny was in 1970, when they appeared in a public service announcement together to promote savings and loans. This was shortly after the whole cast and crew of The Jack Benny Show had joined for Jack Benny’s Twentieth Anniversary Special.

In 1972, he co-starred with June Allyson and Judy Canova in the First National Tour of the Broadway musical No, No, Nanette.

In 1976, Day was the voice of “The Preacher” in the Rankin-Bass production Frosty’s Winter Wonderland and again worked with them in 1978, when he voiced Fred in The Stingiest Man in Town, which was their animated version of Charles Dickens’ novel A Christmas Carol.

Day died on June 22, 1988, of amyotrophic lateral sclerosis (ALS, 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 Culver City’s Holy Cross Cemetery.