“George Brett yesterday took a newspaper from a writer and began reading names aloud to teammates. When asked what he was doing, he answered ‘I’m reading the Mendoza Line.’ He then explained, ‘There’s the Fault Line, the International Date Line—we read the names of the guys below the Mendoza Line: all the guys hitting below .200.’ Blessed be Mario Mendoza.” Peter Gammons, The Boston Globe, May 17, 1982, p26
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1982. “Dave Revering has a lot of nerve demanding to play full-time. He can’t even reach the famed Mendoza Line (.200)” (Al Strachan, The Globe and Mail [Toronto], July 28).