Mary Lou Williams (born Mary Elfrieda Scruggs; May 8, 1910 – May 28, 1981) was an American jazzpianist, arranger, and composer. She wrote hundreds of compositions and arrangements and recorded more than one hundred records (in 78, 45, and LP versions). Williams wrote and arranged for Duke Ellington and Benny Goodman, and she was friend, mentor, and teacher to Thelonious Monk, Charlie Parker, Miles Davis, Tadd Dameron, Bud Powell, and Dizzy Gillespie.

She has been noted for her 1954 conversion to Catholicism, which led to a musical hiatus and a later transformation in the nature of her music. She continued to perform and work as a philanthropist, educator, and youth mentor until her death from bladder cancer in 1981.

In 1927, Williams married saxophonist John Overton Williams. She met him at a performance in Cleveland where he was leading his group, the Syncopators, and moved with him to Memphis, Tennessee. He assembled a band in Memphis, which included Williams on piano. In 1929, 19-year-old Williams assumed leadership of the Memphis band when her husband accepted an invitation to join Andy Kirk‘s band in Oklahoma City. Williams joined her husband in Oklahoma City but did not play with the band. The group, Andy Kirk’s Twelve Clouds of Joy, moved to Tulsa, Oklahoma, where Williams, when she wasn’t working as a musician, was employed transporting bodies for an undertaker. When the Clouds of Joy accepted a longstanding engagement in Kansas City, Missouri, Williams joined her husband and began sitting in with the band, as well as serving as its arranger and composer. She provided Kirk with such songs as “Froggy Bottom”, “Walkin’ and Swingin'”, “Little Joe from Chicago”, “Roll ‘Em”, and “Mary’s Idea”.

Williams was the arranger and pianist for recordings in Kansas City (1929) Chicago (1930), and New York City (1930). During a trip to Chicago, she recorded “Drag ‘Em” and “Night Life” as piano solos. She used the name “Mary Lou” at the suggestion of Jack Kapp at Brunswick Records. The records sold quickly, raising Williams to national prominence. Soon after the recording session she became Kirk’s permanent second pianist, playing solo gigs and working as a freelance arranger for Earl Hines, Benny Goodman, and Tommy Dorsey. In 1937, she produced In the Groove (Brunswick), a collaboration with Dick Wilson, and Benny Goodman asked her to write a blues song for his band. The result was “Roll ‘Em”, a boogie-woogie piece based on the blues, which followed her successful “Camel Hop”, named for Goodman’s radio show sponsor, Camel cigarettes. Goodman tried to put Williams under contract to write for him exclusively, but she refused, preferring to freelance instead.