In 1969 he won All-American honors and began his professional career as a catcher in the Montreal Expos organization where he played under the name John Reen.
Hart then coached baseball at William R. Boone High School in Orlando, Florida before joining the Orioles organization in 1982 as a minor league manager.
They won six of the seven possible American League Central division titles, which was introduced for the 1994 strike shortened season (1995–99 and 2001) with appearances in the World Series in 1995 and 1997.
On February 15, 2004, the Rangers traded Alex Rodriguez (the AL Most Valuable Player of the 2003 season and only the second MVP on a last place team) to the New York Yankees for Alfonso Soriano and Joaquín Árias.
In addition, the contract stipulated that once it was terminated by either side, it automatically converted to a five-year agreement for Hart to serve as senior advisor to the owner.
After other teams showed interest in hiring Hart as their general manager during the 2005 off-season, the Rangers extended his senior advisor contract for three more years.
[7] The team, at the time stuck in the lower reaches of the National League East Division, was in the midst of a "reset" which saw them significantly strengthen their young pitching talent, though the rebuild came at a cost.
The probe resulted in the resignation and subsequent lifetime ban of Hart's subordinate, general manager John Coppolella, who was later reinstated, and the one-year suspension of the club's scouting director.