He made his MLB debut for the White Stockings in 1887, when a writer for the Oshkosh Daily Northwestern called him the "swiftest pitcher in the National League" (NL).

Released by Chicago player–manager Cap Anson, he signed with the Columbus Solons of the American Association (AA) in 1889, where he led the league in innings pitched (513+2⁄3), losses (34), strikeouts (368), and walks (274).

[17] According to a Pittsburgh Daily Post writer, Duluth won its league pennant due "chiefly on account of Baldwin's pitching".

[23] Baldwin listed his birthdate when he played for Chicago in 1887 as 1867, which followed a theme of childishness and "extreme petulance" in him according to baseball historian David Nemec.

[11] In spring training prior to the 1887 season, a hotel employee found Baldwin and Tom Daly unconscious in their room, which smelled of gas.

[29] In June, a correspondent for The Clipper complimented Baldwin on his endurance and curveballs,[30] while an Oshkosh Daily Northwestern writer called him the "swiftest pitcher in the National League".

[33] With the sale of Clarkson to Boston, only White Stockings' player–manager Cap Anson, according to one prediction, believed the team could win a pennant.

[39] A monkey bit Baldwin on a ship on the tour after he fed it pretzels and beer;[40] the bite has been reported in a different source as a serious scratch.

[41] On April 24, 1889, Anson released him and three other White Stockings and stated he would rather "take eighth place with [a team of gentlemen] than first with a gang of roughs";[42] according to Baldwin, Chicago did not restrict alcohol consumption on the tour (team owner Albert Spalding was a "temperance crusader"),[43] and at various banquets Baldwin attended the wine "flowed freely".

[44] In addition to his alcohol consumption on the tour, Baldwin stated that after he hinted he would not sign for the salary of the previous season, he was released.

[46] "A year ago when Spalding released him, [Baldwin] declared that the ambition of his life was to play in opposition to Anson's team.

Baldwin, who debuted for the Solons on May 4 in a showing described by The Chicago Tribune as "anything but credible,"[48] explained his poor opening game as a result of unfamiliarity with AA coaching methods.

[49] By late June, a month in which he hit a double, three triples, and a home run over a three-game span,[50] Baldwin was "doing better", according to a writer for The Chicago Tribune.

[1][3][26] Baldwin's 274 walks set a then-MLB record,[11] while his strikeout total is the most post-1886 except for Nolan Ryan, Sandy Koufax, and Randy Johnson.

[66] In early March, Baldwin tried to convince Jack O'Connor of the Solons and Silver King of St. Louis to sign with Pittsburgh.

[71] Baldwin had a 2.76 earned run average (ERA) and a 1.40 walks plus hits per inning pitched (WHIP) rate in his first year with the club as the Pirates finished last in the NL;[b][73] Baldwin's 23 hit batsmen led the league, while his totals in innings pitched, wins, losses, and complete games led the club.

[76] In July, Baldwin asked Brooklyn Bridegrooms president Charles Byrne to trade him to the team in exchange for pitcher Tom Lovett.

[78] Baldwin either asked for a ten-day vacation[79] or the Pirates suspended him for ten games starting around August 6 due to indifferent play.

[82] The Pirates possibly gave him a ten-day notice due to his involvement in the Homestead strike, a labor strike culminating in a battle between workers of the Carnegie Steel Company and members of the Pinkerton detective agency hired by management to introduce strikebreaker workers to the mill.

[83] In September 1892, Carnegie Steel Company Secretary F. T. F. Lovejoy provided information which left Baldwin charged with aggravated riot.

[7][83] Baldwin stated he was at the strike "merely as a spectator", and when the surrender of the Pinkertons occurred[c] he "went to his home in Homestead and in no way aided or abetted the attack on the defenseless prisoners".

[87][88] In the off-season, Baldwin sold real estate and insurance in Homestead, and stated he did not care about returning to baseball[89] after refusing a contract with the Pirates due to low wages.

[98] An article in The Cincinnati Enquirer stated Baldwin could not find a team with which to sign due to his lawsuit against von der Ahe.

[99] A retrospective article in The Washington Post stated Baldwin and King were "marooned" due to their involvement in the Association–League war of 1891,[11] in which clubs from opposing leagues attempted to sign each other's players.

[110] Though an article in The Philadelphia Inquirer stated Baldwin was "certain to stay" with the team on March 31, 1895,[111] the Phillies released him in April due to his drinking.

[115] The Colts granted his release in June,[d] and he signed with the Rochester Browns of the EL for more pay; for Pottsville, he won over two-thirds of the games in which he pitched according to The Allentown Leader.

He graduated with a Doctor of Medicine degree from Baltimore Medical College in 1900 and practiced in Rochester, Minnesota, in Columbus, and at Passavant Hospital in Pittsburgh.