Mackin is the son of Louis and Aileen Tanner Mackin and grew up on Birmingham's Southside. He earned his bachelor's degree in industrial management at Auburn University and entered the U.S. Navy in 1954 for four years of active duty. He continued as a member of the U.S. Naval Reserve, retiring as a Commander in 1961.
Mackin continued his studies at the Stonier Graduate School of Banking and the Commercial Lending School at the University of Oklahoma and joined the commercial loan department of the Exchange Security Bank in 1966. He had been promoted to head of that division before July 13, 1971, when the bank merged into First Alabama Bank.
Mackin continued to head the commercial loan division while rising through the ranks as vice president, senior vice president, executive vice president, and senior executive vice president. In 1983 he was elected president and chief executive officer of the bank's central region. In 1986 he was elected chairman of the board for the Birmingham bank, serving until January 1990, when he assumed the duties of president and CEO for the parent corporation. In August of that year he was elected chairman and chief executive officer of Regions Financial Corporation.
Mackin stepped down as Regions' CEO in 1998 and retired from the board in May 2001. He also served on the boards of Alabama Public Television Foundation, the Better Business Bureau of North Central Alabama, Auburn College of Business and the Seton Health Corporation of Troy, New York. He also served on numerous civic boards, including Birmingham Operaworks, John Carroll Foundation, Boy's Club of Birmingham and Jefferson County Boy Scouts.
Mackin is a member of the Kiwanis Club of Birmingham. He was also a member of the Leadership Birmingham class of 1984 and Operation New Birmingham. The Auburn College of Business awarded him as "Distinguished Alumnus" in 1991 and Regions' board endowed the J. Stanley Mackin Eminent Scholar Chair at the college in his honor in 1996. Mackin was inducted into the Alabama Academy of Honor in 1998, the Birmingham Business Hall of Fame in 2001, and the Alabama Business Hall of Fame in 2005.
- Gerdes, Bill (August 5, 2005) "Alabama Business Hall of Fame to Induct Five New Members UA News