Mark McGwire

Frank Robinson, Cincinatti Reds 1956 in the NL
Fred Lynn, Boston Red Sox, 1975 in the AL

Tom Alston, who played his first game for them on April 13, 1954.

vernen wells

Bobby Bonilla

The Cubs have the best record and get the Dodgers with the worst record of the teams getting into the playoffs. However the Angels have the best record in the AL and get the Red Sox who have a better record than the eventual AL central winner. Is it based on winning your division, or wild card, or record? Or is it just whoever MLB chooses?

The answer to this question has always been in dispute.
Because the first person on the field prior to game time was an umpire. But, he was a baseball player before he retired and then became an umpire.
So I guess that question could be "the first umpire and ball player" were the same person. And then it depends on which Record Book you believe. One says Billy 'Big Boy' Phillips, and another says, Robert 'Bullet Bob' Pendergrass.
Take your choice.
Here are a few 'firsts' from by 'Book of Firsts':
1st homerun NL - Ross Barnes, Chicago, May 2, 1876 and Charlie Jones, Cincinnati, May 2, 1876
AL - Erve Beck, Cleveland, April 25, 1901
1st pinch-hit grandslam NL - Mike O'Neill, St. Louis, June 3, 1902
AL - Marty Kavanagh, Cleveland, Sept. 24, 1916
1st lefthanded catcher - Bill Harbidge, Hartford, May 6, 1876
1st lefdthanded pitcher - Bobby Mitchell, Cincinnati, 1878
1st pitcher to wear glasses - Will White, Boston, 1877
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jackie robinson

It was Nolan Ryan in 1979.

who was the first person to hit a homerun at turner field the atlanta braves new ball field?