“Baseball’s greatest living players, headed by the great Ty Cobb and Rogers Hornsby, took over the town [New York] today as the National League celebrated its 75th birthday at the Broadway Central Hotel.” Joe Reichler (Associated Press), “Greatest Living Ball Players Celebrate National League’s 75th Birthday Today,” Jackson (TN) Sun, February 2, 1951, p7 (Syndicated story appeared in many papers)
NOTE: The term was used much earlier to refer to players still playing, including Lajoie, Wagner, and Cobb early in the 20th century. The above quote was the earliest use of the term to mean greatest living player of all time.
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
The Dictionary does not cite a first use but notes that the title was “bestowed on Joe DiMaggio in 1969 by an unofficial vote of baseball writers during the celebration of baseball’s centennial.”