According to one account of the early days of professional baseball in Saint John, the Shamrocks went shopping for a new pitcher in 1890: "They obtained the services of F.J. Sexton from Brown University.

During the 1895 season, Sexton played Major League Baseball as a pitcher for the Boston Beaneaters where he was re-united with his battery-mate from Brown, Fred Tenney.

In his Major League debut on June 21, 1895, Sexton allowed only two earned runs, but the Beaneaters were defeated by Brooklyn by a score of 4–2.

Five singles and one double were made by the visitors, and except for an excusable error by Long in the first inning, the Giants would have left town last evening with a shutout.

Tenney and Sexton worked well, the latter holding his man at first in fine style and pitching a cool, heady game.

As both Tenney and Sexton can hit and run, it made Boston by all odds the fastest nine ever put on a ball field.

[17] In the end, the author wrote of the new twirlers: "Stocksdale, Wilson, Yerrick and Sexton were more or less frosts, and were pitched in but few games.

[19][20] In February 1895, The Michigan Alumnus announced, "F.G. [sic] Sexton, Brown University's crack pitcher is a student here this year and will help with the nine.

In January 1896, The Michigan Alumnus announced: "Sexton, who pitched last season and who was change pitcher on the Boston league team last summer, has been engaged as coach and will not try for his old position.

The Michigan Alumnus reported on the incident as follows:"In the second half of the seventh inning, with the score 11–11, one man out and another on base, the Indiana pitcher gave out and the captain noticed this and gave his umpire the tip to call the game, which he did, giving darkness as his reason, though the teams might easily have played a half hour or more longer.

While continuing his medical practice, Sexton served as the baseball coach at his alma mater, Brown University.

In 1907, the Brown Alumni Monthly noted:"Very much of this year's creditable showing is due to Frank J. Sexton, the coach, who has a great faculty for the developing of new baseball material.

[13][30] Upon his arrival, Sexton promptly instituted a new training regimen, requiring the pitchers and catchers to do daily work on the gymnasium apparatus for a month before regular practice began.

Sexton told the press that the work would get their arms in shape "to avoid strains and other hurts when they begin throwing the ball around.

"[31] In early March 1911, Sexton signed a two-year contract agreeing to coach the Harvard baseball team through the 1912 season.

In addition to his requiring the battery candidates to work on the gymnasium apparatus, he instituted a rigorous program of fall practice for the baseball team.

"[34] In 1915, a publication known as The World's Advance profiled a new target device used by Sexton in the training of Harvard's pitching staff.

[37] In a letter published by The Boston Evening Transcript, Sexton wrote that he did not agree to the many suggestions of the committee and concluded, "After full consideration of the matter discussed, I felt I could not retain my self-respect and esteem owing to their persistent interference and continue as coach of the Harvard University baseball team.

"[38] The Evening Transcript wrote that Sexton was "considered one of the most efficient in the country" and opined that he "has been a good coach for Harvard and has developed some excellent teams from very ordinary material.

[41] During a jury trial in March 1917, Sexton gave a demonstration of his methods, placing a handkerchief on the floor as home plate, and explaining the position from which he wanted the catcher to throw the ball.

Sexton testified that the person who the Athletic Committee sought to impose as an assistant coach wanted to override his instruction to the catchers.

[42] At the time of the 1920 Census, Sexton was living at 1032 Commonwealth Avenue in Brookline, Massachusetts with his wife Louise (age 44).