Hardiman is a private investor and director of several for-profit and nonprofit enterprises, including Brown Advisory Mutual Funds, Florida Coalition for Coastal Preservation, and the Baltimore Chesapeake Bay Outward Bound Center. He also has served on the boards of a number of public companies and mutual fund organizations, including Franklin Resources, Inc, Corvis Inc., Broadwing, Inc, the Scudder Funds, the ISI Funds, and the Alex. Brown Funds.

Previously, he served as president and chief executive officer of the National Association of Securities Dealers, Inc. and its wholly owned subsidiary, The Nasdaq Stock Market, Inc., from September 1987 through January 1997. Trained as a lawyer with BA and LLB degrees from the University of Maryland Carey School of Law, Hardiman engaged in the practice of general corporate and securities law in Baltimore from 1963 until 1968 when he joined the securities firm of Robert Garrett & Sons, Inc. with whom he was associated as an executive vice president and a director until he joined Alex. Brown & Sons in 1975. He became a general partner of Alex. Brown in 1976 and, on its incorporation, a managing director, chief operating officer, and a member of the Board of Directors until September 1987. 

Prior civic activities included numerous volunteer positions in Baltimore, including Baltimore School for the Arts Foundation, chairman of the Fund for Educational Excellence; trustee of St. Paul’s School for Girls and the Baltimore Museum of Art; and director of the University of Maryland Medical System. In 1991, he received the Securities Industry Association's Travers J. Bell Memorial Award of Distinction. Three years later, Hardiman was the recipient of the President’s Award for the Distinguished Alumnus of the University of Maryland, College Park.In 2010, he was recognized as a Distinguished Alumnus of the University of Maryland Carey School of Law, whose Board of Visitors he chaired for four years. 

Hardiman served on the Board of Directors of the University System of Maryland Foundation for many years, becoming emeritus in 2023. 

He has served on the UMBF Board of Trustees since 2008, chairing the Finance committee from 2016 to 2023.