As Rivera was finishing his mural project at the Ministry of Education in Mexico City, Mrs. Van Horn joined him and other artists in the production of the work.
[4][5] For four years (1928–1932), she and her husband lived in Berkeley, California where she joined various art leagues and worked with prominent artists in the Bay Area, including John Emmett Gerrity, David Park and Galka Scheyer who represented The Blue Four: European abstractionists, Feininger, Kandinsky, Jawlensky and Klee.
In Washington she met Douglas MacArthur, who fell in love with her, as evidenced by a recently discovered cache of letters written to her by him.
Van Horn was also a military officer who subsequently served under MacArthur during the First World War and retired as a brigadier general in 1940.
When Margaret contracted tuberculosis and died in 1932, Van Horn, devastated by the loss, left her family for some time.