
'Notes On Journalism' http://books.google.com/books?id=52L2eI9mwlcC&q="No+one+in+this+world+so+far+as+I+know+and+I+have+searched+the+record+for+years+and+employed+agents+to+help+me+has+ever+lost+money+by+underestimating+the+intelligence+of+the+great+masses+of+the+plain+people"&pg=PA28#v=onepage in the Chicago Tribune ( 19 September 1926 http://archives.chicagotribune.com/1926/09/19/page/87/article/notes-on-journalism)
The first sentence is often paraphrased as "No one ever went broke underestimating the intelligence of the American people." (The Yale Book of Quotations, 2006, p. 512)
1920s
Source: Gist of Mencken