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 briskly, 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. In 1942, Williams, who had divorced her husband, left the Twelve Clouds of Joy, returning again to Pittsburgh. She was joined there by bandmate Harold "Shorty" Baker, with whom she formed a six-piece ensemble that included Art Blakey on drums. After an engagement in Cleveland, Baker left to join Duke Ellington's orchestra. Williams joined the band in New York City, then traveled to Baltimore, where she and Baker were married. She traveled with Ellington and arranged several tunes for him, including "Trumpet No End" (1946), her version of "Blue Skies" by Irving Berlin. She also sold Ellington on performing "Walkin' and Swingin'". Within a year she had left Baker and the group and returned to New York. Williams accepted a job at the Café Society Downtown, started a weekly radio show called "Mary Lou Williams's Piano Workshop" on WNEW and began mentoring and collaborating with younger bebop musicians such as Dizzy Gillespie and Thelonious Monk. In 1945, she composed the bebop hit "In the Land of Oo-Bla-Dee" for Gillespie. "During this period Monk and the kids would come to my apartment every morning around four or pick me up at the Café after I'd finished my last show, and we'd play and swap ideas until noon or later", Williams recalled in "Melody Maker".