Nicholson's mother was of Irish, English, and German descent.
Nicholson was born on April 22, 1937, in Neptune City, New Jersey, the son of a showgirl, June Frances Nicholson (stage name June Nilson). She married Italian-American showman Donald Furcillo (stage name Donald Rose) in 1936, before realizing that he was already married. Biographer Patrick McGilligan stated in his book Jack's Life that Latvian-born Eddie King (originally Edgar A. Kirschfeld), June's manager, may have been Nicholson's biological father, rather than Furcillo. Other sources suggest June Nicholson was unsure of who the father was. As June was only 18 years old and unmarried, her parents agreed to raise Nicholson as their own child without revealing his true parentage, and June would act as his sister.
In 1974, Time magazine researchers learned, and informed Nicholson, that his "sister", June, was actually his mother, and his other "sister", Lorraine, was really his aunt. By this time, both his mother and grandmother had died (in 1963 and 1970, respectively). On finding out, Nicholson said it was "a pretty dramatic event, but it wasn't what I'd call traumatizing... I was pretty well psychologically formed".