Gary was born Aug. 31, 1937, in Flandreau, South Dakota, a rural farm town outside of Sioux Falls, to Eleanora Lemon and James McMahon. As a youth, Gary excelled in academics and sports, graduating from Flandreau High School in 1955. In 1958, he married the love of his life and best friend, Shirley Hahn, a childhood schoolmate. Eager to make something of himself, Gary applied to the Univ. of South Dakota in 1959. He graduated with honors with a Bachelor of Science degree in accounting, and would later be named "Distinguished Alumnus" by the universitys business school in 1973. Having caught the attention of his professors, Gary was encouraged to pursue graduate study and in 1960, was awarded a fellowship to the University of Wisconsin, where he obtained a Masters in Business Administration in finance, with a 4.0 grade point average. While completing his education, Gary also served in the U.S. Army Reserves as an artillery captain. Having completed his degrees and military requirements, it was finally time to start his career! With Shirleys support and dreams of a new life in the west, Gary joined the Denver office of Haskins & Sells accounting firm in 1960. Gary went on to pass the CPA exam in 1962 and was promoted to partner in 1971. Eventually, the firm took notice of his work ethic and approachable professional style, making him managing partner of the Denver office from 1977-1983. From 1983-1989, Gary served as managing partner of the western region of the U.S. for what was now Deloitte, Haskins & Sells. While in Denver, Gary was a devoted contributor to the Boys & Girls Clubs of Metro Denver; Rotary Club of Denver, Junior Achievement and Outward Bound. Gary and Shirley were members of St. Joan of Arc Catholic Church in Arvada, the Lakewood YMCA and Denver Country Club. They owned a condo in Vail, where they spent many family vacations and were loyal fans of the Denver Broncos football team. In 1989, Deloitte Haskins & Sells and Touche Ross & Co. completed a merger and Gary was named group managing partner and Dallas office managing partner, a position requiring the family to move to Texas. Gary retired from Deloitte & Touche in 1997, having served 37 years with the company. While in Dallas, he held board positions at the Greater Dallas Chamber, Dallas Theater Center, Dallas Citizens Council, Dallas Business Committee for the Arts and the North Texas Commission. Upon retirement in 1997-98, Gary was named Chairman of the Board of the United Way of Metropolitan Dallas, a civic position he was perhaps most proud of, as he always felt anyone could be successful, given the opportunity for education and advancement. A product of a broken home and economic hardship as a youth, Gary embodied that ideal and welcomed the chance to better the lives of young people. In 1999, at the age of 62, Gary received the news nobody wants to hear. He was diagnosed with Parkinsons disease, an incurable disorder of the central nervous system, affecting movement. He took on "his toughest competitor" like a champ, choosing a proactive approach to coping with the disease, volunteering for studies at the National Institute of Health in Maryland and participating in local support groups. As the years passed, Gary continued to pursue his passions and hobbies in retirement, rarely if ever complaining about his condition. He loved exercising, cooking out and watching sports with his wife and children, swimming with his grandchildren; golfing with his buddies at the Sports Club in Las Colinas, and reading the sports page, a tradition he continued until his last days. In the end, Gary died peacefully at home on Saturday, May 23, 2020 surrounded by his family. Survivors include his daughter, Michele of Frisco (Randy); son Michael of Dallas (Tina); and four grandchildren: Amanda, Chrissy, Ellie and Hatcher. He is preceded in death by his parents; his brother, Thomas McMahon of Sioux Falls, and his daughter, Callae McMahon of Irving, TX. Memorial services will be 3:00 p.m. Friday, December 3, 2021 in the chapel of Colonial Funeral Home of Irving, 606 West Airport Freeway 75062. In Garys honor and in lieu of flowers, the family welcomes donations to the United Way of Metropolitan Dallas via the link: www.UnitedWayDallas.org/Donate or the Parkinsons Foundation at: www.parkinson.org The service will be live-streamed and can be viewed at the following link: https://view.oneroomstreaming.com/authorise.phpk=1638329272151227