Plas John Johnson Jr. was born on July 21, 1931, in Donaldsonville, Louisiana. He first sang with his family's group, and his father, a working saxophonist who also played banjo, bought him a soprano saxophone. Largely self-taught, he moved to alto and then to tenor, and he later said he picked up his early phrasing by matching his father's tone. With his brother Ray, a pianist, he made his first records in New Orleans in the late 1940s as the Johnson Brothers, and a small-label session backing the singer Erline Harris has been dated to 1949.

In 1951 he began touring with the singer and pianist Charles Brown, and he served in the U.S. Army during the Korean War. After his discharge he and Ray moved to Los Angeles in 1954. Plas enrolled briefly at the Westlake School of Music, but session offers arrived faster than classes, and within about a year he was working as a full-time studio musician. The arranger and producer Maxwell Davis, who was busy producing for the Modern label, hired him to cover his own saxophone parts, and the early R&B dates in Los Angeles drew jazz players moonlighting on blues sessions, among them trumpeter Art Farmer. The bandleader Johnny Otis brought him to Capitol Records, and the label's A&R executive Dave Cavanaugh signed him to a contract.

His own records began in 1956 with a Tampa Records quartet date that featured Ray on piano. At Capitol he cut a run of singles from 1957 to 1959 and then two albums that stand as his best-known leader work, This Must Be the Plas (1959), with pianist Paul Smith, guitarists Howard Roberts and Bill Pitman, bassist Red Callender and drummer Earl Palmer, and Mood for the Blues (1961), with strings arranged by Gerald Wilson and Rene Hall. In 1962 and 1963 he made two albums for the budget label CRC Charter under the name Johnny Beecher, a pseudonym he used to avoid contractual conflicts. For almost twenty years he averaged two sessions a day, playing on records by Peggy Lee, Nat King Cole, Frank Sinatra and Ella Fitzgerald, on Les Baxter's exotica albums, on rock and roll and R&B singles, and on many records by the Beach Boys. He was also part of studio-only groups such as B. Bumble and the Stingers and the Marketts.

In the late 1950s and early 1960s he was a regular in Henry Mancini's studio orchestra, a featured soloist on More Music from Peter Gunn (1959), and in 1963 he recorded the tenor saxophone melody of "The Pink Panther Theme" for the film. Mancini wrote the part with him in mind, and Johnson recalled that it took two takes and that the musicians, string players included, applauded at the end. Neal Hefti also used him and trumpeter Harry "Sweets" Edison on the theme to the television series The Odd Couple.

In 1970 he joined the studio band of The Merv Griffin Show, which drew jazz players including Ray Brown, Herb Ellis, drummer Jake Hanna and trombonist Kai Winding. With Brown, Ellis and Hanna he made the Concord Jazz albums The Blues and Positively in the mid-1970s, and he appeared on Concord's Ellis-Brown sextet dates with Edison. He recorded direct-to-disc albums for Sheffield Labs with the pianist Lincoln Mayorga, played in the Capp-Pierce Juggernaut big band, and toured with the Gene Harris Superband in 1990. His later leader albums included On the Trail! (recorded in Oslo in 1991 with the Norwegian saxophonist Totti Bergh) and Keep That Groove Going! (2001), a tenor meeting with Red Holloway at Rudy Van Gelder's studio with organist Gene Ludwig, guitarist Melvin Sparks and drummer Kenny Washington. He was featured at the 2003 Vail Jazz Festival and appears in the 2008 documentary The Wrecking Crew.

Plas Johnson passed away on July 15, 2026, in Granada Hills, Los Angeles, six days before his 95th birthday. Obituaries in The Guardian and on KJazz 88.1 led with the "Pink Panther" solo.

Johnson's tenor sound was seductive, in AllMusic critic Scott Yanow's word, and more versatile than its Pink Panther fame suggests: equally at home in blues, R&B and hard bop settings. He came up playing jump blues and rhythm and blues, and the same blues phrasing carried into his soul-jazz and hard bop leader dates; AllMusic's related-artist list for him includes Gene Ammons, Cannonball Adderley, Charlie Parker, Illinois Jacquet, King Curtis and Richie Cole. As a studio player he specialized in eight- and twelve-bar solos and in fills behind singers, and he said his solos "always seemed to bring the record up another notch." A large share of his recorded legacy is accompaniment on pop, R&B and film sessions rather than jazz, so the jazz case rests on his leader albums, his Concord and Milestone dates and his work with jazz-based orchestras and big bands. He also played alto, baritone and flute.