I've just pulled up his obituary from the New York Times, published on December 14, 1961.

Here is what I just posted at IMDb:

John Charles Thomas's birth and death dates have been changed by "Dr. Kaiser." Thomas was born Sept. 6, 1891 and died on Dec. 13, 1960.

See Wikipedia (as a starting point)
http://en.wikipedia.org/wiki/John_Charles_Thomas

The _New York Times_ ran an obituary, published Dec. 14, 1960 titled "JohnCharles Thomas Dies at 69; Concert, Opera and Radio Star: _________;___. o Popular Baritone Performed In Musicals and Operettas Noted as Showman" (I suspect the underlines are for fancy text that didn't scan properly.)

Here it is via the ProQuest citation:

JohnCharles thomas dies at 69; concert, opera and radio star. (1960, Dec 14). New York Times (1923-Current File). Retrieved from http://search.proquest.com/docview/115104439?accountid=7117

SRS