As far as minor league pitchers, it is tough for a pitcher to win over 200 games, let alone over 300 games, in their minor league career. The goal of a pitcher is not to set all types of minor league records for longevity, but to make an impact in the major leagues. No pitcher aspires to be just a great minor league pitcher. Similar to the hitters who have hit the most minor league home runs, none of the pitchers who have over 300 minor league wins wanted to remain there. In fact, probably until the pitcher is at the end of the line- does he drop his goals and aspirations to simply focus on his minor league career. While the goal of a player is to help whatever team he is playing for at whatever level win, nobody aspires to be a career minor leaguer.
A LHP by the name of Tony Freitas won a total of 342, which is a record for a left hand pitcher. Freitas holds the distinction of being the only pitcher in the top four with more than one year of MLB experience. He would pitch in over 100 games from 1932-1936 for the Philadelphia Phillies and Cincinnati Reds. The all time leader is a RHP by the name of Bill Thomas who won 383 games from 1924-1952. He was briefly banned for life for an association with gamblers who had a hand in the 1946 Championship Series in the minor leagues. He was reinstated after stating the case he was 5-0 in the playoffs and since there was no proof he received any money, he was allowed to continue to pitch. Thomas holds the difference of being the only pitcher in this group to never pitch in the major leagues. RHP Joe Martina won a total of 349 games in 23 minor league seasons and hadover 2700 Ks, ranking second in the history of the minor leagues. His claim to fame was being part of the 1924 World Series Champion Washington Senators, pitching 24 games, 14 starts and making an appearance in the 1924 Worlds Series against the New York Giants. The pitcher with the 3rd most wins in the history of minor league baseball is George Payne, a RHP who pitched in the minors for 28 years (1913-1940). Payne pitched 12 games for the 1920 White Sox, the year before 8 players were banned for life for throwing the 1919 World Series.