She was the wife of New York School artist Philip Guston, whom she met while attending the Otis Art Institute.

She worked as a painter under the Section of Fine Arts, creating murals in public places during the Great Depression.

[4] Her daughter, Musa Mayer (née Guston), noted that McKim's sister Josephine is not depicted, although she suggests that the "spirited black horse their mother struggles to control" could be a symbolic representation of her.

[10] Guston posed for her husband throughout his career for various works, including a 1944 painting entitled The Young Mother, for which she sat along with their daughter, also named Musa.

She spent much of her young life in Panama, as her father Frederick was employed as a civil servant in the Canal Zone.

[14] In 1930, she met her future husband, abstract expressionist New York School painter Philip Guston (then Phillip Goldstein) while attending the Otis Art Institute.

She sent a letter to the Stanley Rose Book Shop, a Hollywood gathering place of artists, inquiring as to the address of Phillip Goldstein.

[16] Fellow painter Herman Cherry gave the letter to Goldstein, he replied, and McKim returned to Los Angeles to live with him soon after.

[21] In 1949, following a brief stay in Rome, Musa and Philip Guston began commuting between Woodstock and New York City, where the latter opened a studio.