Conrad Henry Kirnon, known professionally as Connie Kay, was born on April 27, 1927, in Tuckahoe, New York. He taught himself to play the drums and by the mid-1940s was working in Los Angeles; AllMusic lists early jobs in that decade with the pianist Sir Charles Thompson, Miles Davis and the trumpeter Cat Anderson. His drumming is preserved on The Hunt, the recording of a July 6, 1947, Los Angeles jam session built around the dueling tenor saxophones of Dexter Gordon and Wardell Gray, with Hampton Hawes on piano and Barney Kessel on guitar, which was not issued until 1977.

Between 1949 and 1955 he worked off and on in the quintet of Lester Young, and in the same years he played with the pianist Beryl Booker, Stan Getz, Coleman Hawkins and Charlie Parker. In the early to mid-1950s he was also a regular on Atlantic Records rhythm-and-blues sessions, including Big Joe Turner's "Shake, Rattle and Roll" and Ruth Brown's "(Mama) He Treats Your Daughter Mean."

In January 1955, on the last night of the Modern Jazz Quartet's Birdland engagement, the drummer Kenny Clarke announced that he was leaving the group. The quartet's manager, Monte Kay (no relation), suggested Connie Kay, who joined the day after. Inside the quartet, pianist John Lewis was musical director, vibraphonist Milt Jackson handled public relations, and bassist Percy Heath handled the finances, while Kay arranged lodging and transportation. His first record with the group was Concorde, made in New York on July 2, 1955, for Prestige. In 1956 the quartet signed with Atlantic, which issued Fontessa that year, and in October 1956 it made its first European trip. Over the next nineteen years Kay recorded with the quartet for Atlantic and, in 1968 and 1969, for the Beatles' Apple label, and he enriched Lewis's chamber-style writing with timpani, small cymbals, triangle, bell tree and darbukas alongside his drum kit.

Away from the quartet he was a sought-after guest on small-group dates: Chet Baker's Chet (six tracks from a December 30, 1958, session), the Milt Jackson and Coleman Hawkins album Bean Bags (1958), the Milt Jackson and Ray Charles album Soul Brothers (1958), The Ivory Hunters with pianists Bob Brookmeyer and Bill Evans (1959), Cannonball Adderley's Know What I Mean? with Evans (1961), Bobby Timmons's Born to Be Blue! (1963) and Sonny Stitt's Stitt Plays Bird (1963). His longest outside partnership was with the alto saxophonist Paul Desmond and the guitarist Jim Hall: First Place Again, recorded in September 1959 for Warner Bros. with Percy Heath, was the first of five studio albums by Desmond quartets that included Hall and Kay, and the RCA Victor albums that followed include Take Ten (1963), Bossa Antigua and Glad to Be Unhappy (both 1964). In September 1974 Kay joined Desmond again, with guitarist Ed Bickert and bassist Ron Carter, for Pure Desmond. In September and October 1968 he played drums on the singer-songwriter Van Morrison's Astral Weeks, recorded in New York, and he later appeared on four songs of Morrison's Tupelo Honey (1971) and one on Saint Dominic's Preview (1972).

Jackson quit the quartet in July 1974, and a farewell concert at Avery Fisher Hall on November 25, 1974, was released as The Last Concert. From 1975 to 1981 Kay worked with pianist Tommy Flanagan, the group Soprano Summit and Benny Goodman, including Goodman's 40th anniversary concert at Carnegie Hall on January 17, 1978, and he was the house drummer at Eddie Condon's club. The MJQ reunited in 1981 for a tour of Japan, and Kay spent his last thirteen years back with it, recording the Pablo albums Together Again: Live at the Montreux Jazz Festival '82 (1982), Echoes (1984) and Topsy: This One's for Basie (1985), then Three Windows (1987) with the New York Chamber Symphony and For Ellington (1988). In 1989 he received an honorary doctorate of music from Berklee College of Music. He had a stroke in 1992, and the drummer Mickey Roker filled in during his recovery on some of the quartet's last recorded tracks; Kay recovered enough to perform again. He died of cardiac arrest in Manhattan on November 30, 1994, at 67. Albert Heath, Percy Heath's brother, took the drum chair in February 1995, and a 1960 concert recorded in Ljubljana was issued the same year as Dedicated to Connie. Kay never recorded as a session leader.

Kay is remembered for restraint, subtlety and an unhurried sense of swing, qualities that suited the Modern Jazz Quartet's blend of bebop, blues and baroque-style counterpoint. His job in that group was to keep time quietly beneath John Lewis's ensemble writing and Milt Jackson's blues-rooted solos, and he said himself that his main asset was good time. Critics singled out his ride cymbal, with its dry, metallic tone and slight sizzle, and the way he could carry a stretch of music on hi-hat and cymbals while barely touching the snare. Where his predecessor Kenny Clarke had grown restless with the group's increasingly orchestrated music, Kay fit it comfortably, and the timpani, finger cymbals, triangle and bell tree he added to the kit became part of the quartet's sound. Outside the group his light, precise touch was in demand with horn players and guitarists, from Lester Young in the early 1950s to Paul Desmond and Jim Hall in the 1960s, and he also worked in rhythm and blues. He was succeeded in the quartet's drum chair by Albert Heath.