Ron Murray, a colleague at the Library of Congress (and no known relation to me), sent me a note about the history of the term(subscription required). The definition of the first sense is “A mixture or fusion of disparate elements” with the notation that usage is rare before the late 20th century, and the OED includes this quotation:
1859D. BOUCICAULT Octoroon I. 13 He don’t understand; he speaks a mash up of Indian, French, and Mexican.
The reference to “Octoroon” appears to be for The Octoroon that was first performed in 1859, making the mashup term about 151 years old.called (This post was updated on 25-Sep-2013.)