The first use of the term "Diploma mill" in the NY Times, Boston Globe, Washington Post or LA Times was on August 17, 1880 in the New York Times. "PHILADELPHIA, Aug. 17.1880--The notorious Dr. John Buchanan, who for several years has been engaged in the manufacture and sale of bogus medical diplomas in this city, brought his wicked career to an untimely end this morning about 1 o'clock by jumping overboard from a …ship." ProQuest offers a search able data base ("Historical newspapers') of these publications and a few more. You can get the entire paper, ads and all, in a PDF format. Dates vary but the NY Times goes back to 1851. Most end in the 1970's.
