Guardian earned a bachelor's degree in history in 1975 from Upsala College in East Orange. His first job after graduation was with the Boy Scouts of America, where he worked until 1980. He was a Claridge Casino Hotel executive from 1991 to 1993 and ran the Casino Reinvestment Development Authority's Special Improvement District from 1992 until 2013. He most recently served as Toms River Township business administrator.

Previously, Guardian was mayor of Atlantic City from 2014 to 2018. In a city with just 10% Republican registration, he became the first Republican mayor in almost 25 years.

He is actively involved in Rotary International, Metropolitan Business and Citizens Association, the Atlantic County Improvement Authority, and the NAACP.

Guardian resides with his husband, Louis Fatato, in Atlantic City.