Gerald Joseph Mulligan (April 6, 1927 – January 20, 1996), also known as Jeru, was an American jazz saxophonist, clarinetist, pianist, composer and arranger. Though primarily known as one of the leading jazz baritone saxophonists—playing the instrument with a light and airy tone in the era of cool jazz—Mulligan was also a significant arranger working with Claude Thornhill, Miles Davis, Stan Kenton, and others. His piano-less quartet of the early 1950s with trumpeter Chet Baker is still regarded as one of the best cool jazz ensembles. Mulligan was also a skilled pianist and played several other reed instruments. Several of his compositions, including "Walkin' Shoes" and "Five Brothers", have become standards.

Gerry Mulligan was born in Queens Village, Queens, New York, the son of George and Louise Mulligan. His father was a Wilmington, Delaware, native of Irish descent; his mother a Philadelphia native of half-Irish and half-German descent. Gerry was the youngest of four sons with George, Phil and Don preceding him.

George Mulligan's career as an engineer necessitated frequent moves through numerous cities. When Gerry was less than a year old, the family moved to Marion, Ohio, where his father accepted a job with the Marion Power Shovel Company.

With the demands of a large home and four young boys to raise, Mulligan's mother hired an African American nanny named Lily Rose, who became especially fond of the youngest Mulligan. As he became older, Mulligan began spending time at Rose's house and was especially amused by Rose's player piano, which Mulligan later recalled as having rolls by numerous players, including Fats Waller. Black musicians sometimes came through town, and because many motels would not take them, they often had to stay at homes within the black community. The young Mulligan occasionally met such musicians staying at Rose's home.

The family's moves continued with stops in South Jersey (where Mulligan lived with his maternal grandmother), Chicago, and Kalamazoo, Michigan, where Mulligan lived for three years and attended Catholic school. When the school moved into a new building and established music courses, Mulligan decided to play clarinet in the school's nascent orchestra. Mulligan made an attempt at arranging with the Richard Rodgers song "Lover", but the arrangement was seized prior to its first reading by an overzealous nun who was taken aback by the title on the arrangement.

When Gerry Mulligan was 14, his family moved to Detroit and then to Reading, Pennsylvania. While in Reading, Mulligan began studying clarinet with dance-band musician Sammy Correnti, who also encouraged Mulligan's interest in arranging. Mulligan also began playing saxophone professionally in dance bands in Philadelphia, an hour and a half or so away.

The Mulligan family next moved to Philadelphia, where Gerry attended the West Philadelphia Catholic High School for Boys and organized a school big band, for which he also wrote arrangements. When Mulligan was sixteen, he approached Johnny Warrington at local radio station WCAU about writing arrangements for the station's house band. Warrington was impressed and began buying Mulligan's arrangements.

Mulligan dropped out of high school during his senior year to pursue work with a touring band. He contacted bandleader Tommy Tucker when Tucker was visiting Philadelphia's Earle Theatre. While Tucker did not need an additional reedman, he was looking for an arranger and Mulligan was hired at $100 a week to do two or three arrangements a week (including all copying). At the conclusion of Mulligan's three-month contract, Tucker told Mulligan that he should move on to another band which was a little less "tame". Mulligan went back to Philadelphia and began writing for Elliot Lawrence, a pianist and composer who had taken over for Warrington as the band leader at WCAU.