“[Tom] Brown is not a heavy hitter, but his fielding is conceded to be of the very best; his base running likewise, and as a lead-off batter there is none on the nine that can fill the position as well as he.” “Talk of the ‘Fans’,” The Star (Washington), February 1, 1896, p21
“‘Yes, President Franklin notified me today that he had made up his mind to give me a trial as manager of the Buffalos, so the next season you will see me as player-manager and lead-off hitter with Jay Andrews or Cy Hooker following.'” George Carey quoted in “George Carey Now Manager,” Buffalo Express, September 25, 1900, p10
“He [Washington manager Jake Stahl] hasn’t a doubtful man on the list and his only trouble now is to select a lead-off man in the batting list.” “Nationals Look Like Winners,” The Star (Washington), March 26, 1906, p9
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
Leadoff (as a synonym of “leadoff batter”): 1906. “[Bill] O’Neill looks like a good lead off, being left handed, fast, and a good waiter.” (Chicago Tribune, March 26; Peter Morris).
Leadoff man: 1910. (Baseball Magazine, April, p.64; Edward J. Nichols).
The Dictionary does not provide earliest uses for “leadoff batter” or “leadoff hitter,” but the uses of those above (and of “leadoff man”) antedate the earliest uses shown in the 2009 edition.
See also “lead off“