Margaret Marian Turner was born on March 20, 1918, in Slough, Berkshire, England, the elder of two daughters of Frank and Janet Turner. She had perfect pitch and played piano by ear from childhood, took violin lessons from the age of nine, and did not have a piano teacher until she was sixteen, which left her a lifelong learner by ear rather than a confident reader of notation. In 1935 she entered the Guildhall School of Music and Drama in London, where she studied piano with Orlando Morgan, won composition scholarships and prizes in 1936 and 1937, and fell for the American jazz she heard on records, including Duke Ellington, Fats Waller, Teddy Wilson and Mary Lou Williams. In 1938 she left Guildhall, against her family's wishes, to join Billy Mayerl's Claviers, a four-piano vaudeville act, billed under the stage name Marian Page. She entertained troops for the Entertainment National Service Association from 1940 and joined the American USO in 1944, touring the continent behind the Allied advance after D-Day.

At a jam session at St Vith, Belgium, on October 14, 1944, she met the Chicago cornetist Jimmy McPartland, a veteran of the Austin High Gang who was then an American soldier assigned to entertain troops, and he asked her to play piano in his sextet. They married on February 3, 1945, in Aachen, Germany. The marriage made her an American citizen alongside her British citizenship, which she kept for the rest of her life, which is why she is described both as an English and as an American pianist. They made their first record together in London on January 6, 1946, and reached New York on April 23, 1946. The couple lived in Chicago until 1949, where she made her American debut that June and played in Jimmy's Dixieland group, and then moved to Manhattan. In 1950 she dropped the name Marian Page for Marian McPartland, and on May 8, 1951, her own trio opened at the Embers on 54th Street. She signed her first record contract, with Savoy, that year.

On February 2, 1952, she opened at the Hickory House on West 52nd Street, a residency that ran until November 1962. Ellington was a regular visitor and told her she played too many notes, advice she took to heart. The drummer Joe Morello joined her trio in 1953 and bassist Bill Crow in December of that year; that group was named Small Group of the Year in the 1954 Metronome poll and made her first Capitol albums before Morello left in late 1956 to join Dave Brubeck. In 1958 she was one of the 57 musicians in Art Kane's photograph "A Great Day in Harlem," and she was one of the last four of them still living when she died. She toured in Benny Goodman's septet in 1963, an uneasy fit, since Goodman did not like her modern harmonies. In the 1960s she turned increasingly to writing for DownBeat, to hosting weekly radio shows of records and interviews, and to jazz education, teaching in school clinics from 1964 and in a 1974 program in poor neighborhoods in Washington, D.C. She and Jimmy McPartland divorced but stayed close, and they remarried in 1991, shortly before his death.

In 1969 she co-founded Halcyon Records with Sherman Fairchild and Hank O'Neal, a label for underrepresented players whose first release was Interplay, a set of duets with the bassist Linc Milliman, and which issued albums until 1979. Her friendship with the songwriter Alec Wilder, whom she met in February 1967, led to Marian McPartland Plays the Music of Alec Wilder (1974). In June 1974 she joined Dick Hyman, Hank Jones and Roland Hanna on Let It Happen for RCA as the four-piano Jazz Piano Quartet, and in November 1974 she played a four-piano concert in Buenos Aires with Teddy Wilson, Earl Hines and Ellis Larkins. She headlined the first Women's Jazz Festival in Kansas City in March 1978, and from 1978 she recorded for Concord, with which she had a long association.

Her most public work began with Marian McPartland's Piano Jazz. Recording began on October 8, 1978, and the program premiered on April 1, 1979, on WLTR in South Carolina before NPR offered it nationally, with Mary Lou Williams as its first guest. The format paired conversation with duets at two pianos, at first with pianists and later with singers, horn players, rock and pop performers and even the author Studs Terkel, and it became the longest-running cultural program on NPR and won a Peabody Award. She marked its 25th anniversary with a live taping at the Kennedy Center. NPR announced on November 10, 2011, that she was stepping down, having last taped a show in September 2010, and her friend Jon Weber took over a successor series, Piano Jazz Rising Stars. Her other late work included a symphonic piece, A Portrait of Rachel Carson, performed in 2007 with the University of South Carolina Symphony Orchestra, and her final studio album, Twilight World (2008). She was named an NEA Jazz Master in 2000, received the Recording Academy's Trustees Award in 2004, entered the National Radio Hall of Fame in 2007 and was made an Officer of the Order of the British Empire in the 2010 New Year Honours. She passed away at her home in Port Washington, New York, on August 20, 2013, at the age of 95, of natural causes.

McPartland began in the Chicago-style Dixieland of Jimmy's bands and found her own voice at the Hickory House, where she settled on the piano, bass and drums trio that was becoming the standard small-group format of the day. She was known for rich, subtle harmony, a fondness for ballads, and an ear that let her follow a guest into unfamiliar keys without losing her place; critics found her playing flexible and hard to pigeonhole. She was a synesthete who associated musical keys with colors. She admired Duke Ellington and Teddy Wilson from her student years and took Ellington's advice to play fewer notes to heart, and she tried to absorb the free jazz of the 1960s even though it was not her natural language. She was also an advocate for women in jazz, writing about the subject in DownBeat from the late 1950s, headlining the 1978 Women's Jazz Festival and helping younger players such as the saxophonist Mary Fettig and the singer Susannah McCorkle. Her tributes to other composers, among them Wilder, Billy Strayhorn and Benny Carter, sat beside her own tunes, including "Ambiance," "Twilight World" and "In the Days of Our Love." Through Piano Jazz she became, for many listeners, the person who explained how improvisation works, and the program brought pianists such as Bill Evans, Dave Brubeck and Brad Mehldau to a general audience.