Matta introduced Motherwell to the concept of "automatic" drawing or automatism, which the Surrealists used to tap into their unconscious. The concept had a lasting effect on Motherwell, further augmented by his meeting with the artist Wolfgang Paalen. Motherwell's encounter with Paalen prompted him to prolong his stay in Mexico for several months, in order to collaborate with him. Motherwell's noted Mexican Sketchbook visually reflects the resulting change: while the first drawings are influenced by Matta and Yves Tanguy, later drawings associated with Motherwell's time with Paalen show more plane graphic cadences and details distinguished from the earlier period. Paalen also introduced Motherwell to André Breton, via a letter. Motherwell's seminal trip to Mexico has been described as a little-known but important factor in the history and aesthetics of abstract expressionism. In 1991, shortly before his death, Motherwell remembered a "conspiracy of silence" regarding Paalen's innovative role in the genesis of abstract expressionism.
Motherwell died in Provincetown, Massachusetts on July 16, 1991. On his death, Clement Greenberg, champion of the New York School, left in little doubt his esteem for the artist, commenting that "although he is underrated today, in my opinion he was one of the very best of the abstract expressionist painters".
On July 20, 1991, several hundred people attended a memorial service for Motherwell on the beach outside his Provincetown home. Among them were the writer Norman Mailer and the photographer Joel Meyerowitz, both Provincetown summer residents. Speakers included the poet Stanley Kunitz, who read a poem that was a favorite of Motherwell's, William Butler Yeats's Sailing to Byzantium. Others in attendance included family members, friends, other artists, and Senator Howard Metzenbaum, an acquaintance of Motherwell's.