Ford was born at the Swedish Covenant Hospital in Chicago, Illinois to Christopher Ford (born John William Ford), an advertising executive and former actor, and Dorothy (nee Nidelman), a former radio actress. His father is Irish and his mother is Jewish. A younger brother, Terence, was born in 1945. Ford's paternal grandparents, John Fitzgerald Ford and Florence Veronica Niehaus, were of Irish Catholic and German descent, respectively.

In 1964, after a season of summer stock with the Belfry Players in Wisconsin, Ford traveled to Los Angeles to apply for a job in radio voice-overs. He did not get it, but stayed in California and eventually signed a $150-a-week contract with Columbia Pictures' New Talent program, playing bit roles in films. His first known role was an uncredited one as a bellhop in Dead Heat on a Merry-Go-Round (1966). There is little record of his non-speaking roles (or "extra" work) in film. Ford was at the bottom of the hiring list, having offended producer Jerry Tokovsky after he played a bellboy in the feature. He was told by Tokovsky that when actor Tony Curtis delivered a bag of groceries, he did it like a movie star; Ford felt his job was to act like a bellboy. Ford managed to secure other roles in movies, such as A Time for Killing (The Long Ride Home), starring Glenn Ford; George Hamilton; and Inger Stevens.  His speaking roles continued next with Luv (1967), though he was still uncredited. He was finally credited as "Harrison J. Ford" in the 1967 Western film A Time for Killing, but the "J" did not stand for anything, since he has no middle name. It was added to avoid confusion with a silent film actor named Harrison Ford, who appeared in more than 80 films between 1915 and 1932 and died in 1957. Ford later said that he was unaware of the existence of the earlier actor until he came upon a star with his own name on the Hollywood Walk of Fame. Ford soon dropped the "J" and worked for Universal Studios, playing minor roles in many television series throughout the late 1960s and early 1970s, including Gunsmoke, Ironside, The Virginian, The F.B.I., Love, American Style, and Kung Fu. He appeared in the western Journey to Shiloh (1968) and had an uncredited, non-speaking role in Michelangelo Antonioni's 1970 film Zabriskie Point as an arrested student protester. Not happy with the roles being offered to him, Ford became a self-taught professional carpenter to support his then-wife and two young sons.  Casting director and fledgling producer Fred Roos championed the young Ford and secured him an audition with George Lucas for the role of Bob Falfa, which Ford went on to play in American Graffiti (1973). Ford's relationship with Lucas would profoundly affect his career later on. After director Francis Ford Coppola's film The Godfather was a success, he hired Ford to expand his office and gave him small roles in his next two films, The Conversation (1974) and Apocalypse Now (1979); in the latter film he played an army officer named "G. Lucas".

How long did it take him to rise to fame?