The musical and spiritual life of Mary Lou Williams (1910 - 1981) came together in her later years in the writing of Mary Lou's Mass. Being both Roman Catholic and a jazz pianist and composer, it was inevitable that Williams would be the first jazz composer to write a setting of the mass. The degree of success resulting from the combination of jazz and the traditional forms of Western art music has always been controversial. Because of Williams's personal faith and aesthetics of music, however, she had little choice but to attempt the union of jazz and liturgical worship. After a biography of Williams, discussed in the context of her musical aesthetics, this thesis investigates the elements of conventional mass settings and oral tradition found in Mary Lou's Mass.