While working with John Holmes, Sexton encountered Maxine Kumin. They became good friends and remained so for the rest of Sexton's life. Kumin and Sexton rigorously critiqued each other's work and wrote four children's books together. In the late 1960s, the manic elements of Sexton's illness began to affect her career, though she still wrote and published work and gave readings of her poetry. She collaborated with musicians, forming a jazz-rock group called Her Kind that added music to her poetry. Her play "Mercy Street", starring Marian Seldes, was produced in 1969, after several years of revisions. Sexton also collaborated with the artist Barbara Swan, who illustrated several of her books.