Mark grew up in Virginia, Minnesota and graduated from the University of Minnesota-Duluth in 1972. After earning his wings as a naval aviator in 1973 Mark commanded Navy long range aircraft that served as a communication links between the Joint Chiefs of Staff and our nation’s nuclear submarines. Upon honorable discharge from active duty in 1978, Mark remained in the naval reserve and returned to Minnesota where he began a 31 year career as an airline pilot—retiring from Delta Air Lines in 2010 as a Senior Captain on the Airbus A-330 flying routes to-and-from Europe and Asia.
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Concurrent with his airline career, Mark attended William Mitchell College of Law in St. Paul, Minnesota where he graduated with academic honors in 1986. Following admittance to the State Bar of Minnesota the year of his graduation Mark, his wife Suzanne, and their two children, moved to Brownsville, Texas where Mark practiced law with his father, Paul O’Leary. Mark was admitted to the State Bar of Texas in 1987. In 1991, the O’Leary’s made their move to Sioux Falls, South Dakota where Mark opened his solo practice and continued his law practice in out-state Minnesota. Mark was  admitted to the State Bar of South Dakota in 1994.
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Mark is a current member of the South Dakota, Minnesota, and Texas state bar associations. He is also admitted to practice in several Federal Courts, including those in South Dakota, Minnesota, and Texas.