Dean Martin was born in Steubenville, Ohio and was the younger son of two Italian immigrants, Angela and Gaetano Crocetti. He was named Dino. Until the age of five when he started school, he could only speak Italian. He attended elementary school in Steubenville and then high school, but dropped out in the 10th grade. As a teenager he started to play drums as a hobby. At fifteen he took up boxing and called himself Kid Crochet. He won twenty-five out of thirty-six fights. During these games he broke his nose and his knuckles several times and got a split lip. For a while he was roommate with Sonny King, who was also trying to make his way in showbiz and didn't have much money. It's said they had boxing fights in their apartment and kept going until one of them was done, with people paying to watch.

After a while, Dino quit boxing. He worked in a tobacco shop and an illegal casino while playing with local bands under the name of Dino Martini, after Nino Martini, a famous tenor at the Metropolitan Opera at that time. His first real singing job was for the Ernie McKay Orchestra. In the early '40s, he started singing for Sammy Watkins, who suggested the name of Dean Martin.

In 1941 he married Betty McDonald. They had four children together and got divorced nine years later. They had a troubled relationship marked by Betty's drinking problems, Dean's infidelities and rumored physical abuses. During the '40s he worked for several bands while trying to develop a personal style. He flopped in Riobamba in 1943 when he followed Frank Sinatra.

He was in the army in 1944 during World War II. He served for one year and then he was discharged, possibly because of a double hernia.

By 1946, he had only reached the level of a nightclub singer who succeeded to draw large audiences to the clubs where he played, but no more than that. His singing style had nothing special yet and didn't inspire much compared to the one of Frank Sinatra's.