Julius Brian Watkins was born October 10, 1921, in Detroit, Michigan, and started on French horn at age nine, an unusual choice of first instrument that would define his entire career. He came up playing trumpet rather than horn professionally at first, touring with the Ernie Fields Orchestra from 1943 to 1946 and with pianist Milt Buckner's band from 1949 to 1950. Somewhere in that stretch he also studied for three years at the Manhattan School of Music after settling in New York, and by the late 1940s he was already turning up on record dates led by Kenny Clarke and Babs Gonzales, taking occasional French horn solos on sessions including Three Bips and a Bop's 1949 Blue Note single "Capitolizing" and "Professor Bop." From 1950 on he played French horn exclusively, effectively inventing a role in jazz that had not existed before him: no one had built a career as an improvising soloist on an instrument designed for orchestral blending, with no keys, a treacherous overtone series, and none of the jazz vocabulary that had accumulated on trumpet or saxophone.

Bassist Oscar Pettiford became Watkins's most important early patron, using him in the New Oscar Pettiford Sextet in the early 1950s and then in the two-volume big band album Oscar Pettiford Orchestra In Hi-Fi (ABC-Paramount, 1956 and 1958), where Watkins shared the horn chairs with fellow pioneer David Amram. Watkins led his own record dates for the first time on August 8, 1954, and March 20, 1955, at Rudy Van Gelder's Hackensack studio, cutting two 10-inch Blue Note LPs, New Faces - New Sounds (BLP 5053) and Volume Two (BLP 5064), as the Julius Watkins Sextet with Frank Foster and then Hank Mobley on tenor, Oscar Pettiford on bass, and Kenny Clarke or Art Blakey on drums. Around the same period he was a first-call horn on other bandleaders' dates, including Gigi Gryce's October 1955 Savoy session later issued as Nica's Tempo (where he appears alongside Thelonious Monk and Cecil Payne) and a 1954 Prestige date credited to Monk and Sonny Rollins.

In 1956 Watkins formed a working co-led group with tenor saxophonist Charlie Rouse, first billed as the Rouse-Watkins Jazz Modes and then simply Les Jazz Modes, built around Watkins's horn and Gildo Mahones's piano trio with harpist Janet Putnam and, on several tracks, vocalist Eileen Gilbert. The group recorded four albums for Dawn between 1956 and 1957, Les Jazz Modes (Dawn DLP 1108) and Mood in Scarlet (Dawn DLP 1117) among them, then moved to Atlantic for The Jazz Modes Play Excerpts from the Frank Loesser Musical "The Most Happy Fella" (1958) and the group's self-titled Atlantic album (recorded October 1957 and November 1958, released 1959), which added baritone saxophonist Sahib Shihab to the front line. Watkins and David Amram remained the only two jazz musicians working regularly on French horn through the decade, and on May 18, 1957, at Van Gelder's studio, the two of them co-led a session with pianist Hampton Hawes, trombonist Curtis Fuller, bassist Addison Farmer, and drummer Jerry Segal, supervised by Teddy Charles. Prestige paired those six tracks with a baritone-saxophone session cut by Pepper Adams and Cecil Payne one month earlier, on April 20, 1957, with John Coltrane on tenor and Mal Waldron on piano, and released the combined program in August 1958 as the 16 2/3 rpm sampler Modern Jazz Survey 2: Baritones and French Horns (Prestige 6). The baritone side was reissued in November 1963 under Coltrane's name as Dakar; the French horn side, with Watkins and Amram, was reissued in 1965 as Curtis Fuller and Hampton Hawes with French Horns (Status ST 8305).

The Jazz Modes dissolved around 1959, and Watkins spent the following years absorbed into the busiest rooms in New York studio jazz. Gil Evans put him in the French horn section, alongside Gunther Schuller and Willie Ruff, for Miles Davis's Porgy and Bess (recorded across four sessions from July to August 1958). He toured and recorded with Quincy Jones's big band from 1959 to 1961, including the album The Great Wide World of Quincy Jones and European dates that survive on later live releases, and in January 1962 he led his own final studio date, French Horns for My Lady (Philips), produced by Jones and pairing his horn with three others (John Barrows, Jim Buffington, Bob Northern) plus Gunther Schuller, tuba, and a rhythm section including George Duvivier and Ray Barretto. He was one of four French hornists, alongside Bob Northern, Robert Swisshelm, and Donald Corrado, that Eric Dolphy assembled for Coltrane's Impulse! debut Africa/Brass (May 23 and June 4, 1961), and he appears on Randy Weston's Uhuru Afrika (1961) and Oliver Nelson's Afro/American Sketches (1962). He also documented Charles Mingus's September 1965 UCLA concert and, later, the two-LP Let My Children Hear Music (1972), and he joined the free-jazz collective billed as The Jazz Composer's Orchestra for its 1968 self-titled JCOA album. In the 1970s he became a regular presence in Thad Jones and Mel Lewis's orchestra on record, heard on Consummation (1970), Suite for Pops (1972), and Suite for Pops follow-up New Life (1976), and he played on Mary Lou Williams's Mary Lou's Mass (1975).

Watkins's health declined through the 1970s under the weight of diabetes, liver and kidney problems, and chronic alcoholism. He died of a heart attack on April 4, 1977, in Short Hills, New Jersey, at age 55. Discogs credits him on 444 releases under his own name (7 as leader or co-leader, 411 sideman "Appearance" credits, 5 track-level credits, and 21 later unofficial/bootleg pressings), a tally that understates his actual leader catalog: seven more of his own albums, the two Blue Note 10-inches and all five Rouse-Watkins/Jazz Modes LPs, are filed on Discogs only under group names ("Julius Watkins Sextet," "The Jazz Modes," "Rouse-Watkins Jazz Modes") rather than his personal artist page.

AllMusic calls Watkins "virtually the father of the jazz French horn," and the claim is not much of an exaggeration: before him, the instrument had essentially no improvising vocabulary in jazz, and the two players who built one, Watkins and David Amram, did it in the same handful of New York rooms within a few years of each other. Watkins's solution was to play the horn with the warmth of a trombone and something close to the fluidity of a trumpet, running bebop lines through an instrument with no keys and a notoriously unforgiving overtone series. He was flexible enough to lead a plush, mood-driven co-op with Charlie Rouse's tenor one year and to hold down a chair in Eric Dolphy's dense Africa/Brass horn section or Thelonious Monk's hard-swinging quintet the next, which is largely why bandleaders as different as Oscar Pettiford, Gil Evans, and Quincy Jones all kept calling him for three decades. DownBeat's international critics gave him the Miscellaneous Instrument category two years running, 1960 and 1961, a recognition of the fact that he had no real competition; even after other hornists (Bob Northern, Jim Buffington) began doing studio work, none built the same identity as an improvising voice. His direct influence runs to every subsequent jazz French hornist, and later players such as Vincent Chancey have recorded explicit tributes to him.