Born and raised in Sheboygan, Dennis Grzezinski received his bachelor's degree, magna cum laude, from Princeton University, and his law degree from Yale Law School, where his clinical work included internships with the New Haven Legal Aid Bureau and with the Center for Advocacy, Research & Planning (the legal program of the New Haven Branch of the NAACP).

Mr. Grzezinski has practiced law in Milwaukee for the last 35 years, first as Judicial Law Clerk to Judge Myron Gordon at the United States District Court, and then as a civil litigator with the Frisch Dudek law firm for 14 years, where he tried numerous cases to juries and to state and federal courts. Subsequently he developed his solo practice in the fields of environmental law and civil litigation, primarily representing environmental organizations, citizens groups, municipalities, and individuals throughout Wisconsin. Among his clients at various times have been ACHOICE (now Community Shares of Greater Milwaukee), the ACLU of Wisconsin Foundation, Citizens for a Better Environment, the Door County Environmental Council, Highway J Citizen Group, the Lake Michigan Federation (now Alliance for the Great Lakes), Neighbors Standing United, Preserve Our Parks, the Sierra Club, Waukesha County Environmental Action League, and Wisconsin's Environmental Decade (now Clean Wisconsin). Mr. Grzezinski served as a Commissioner of the Milwaukee Metropolitan Sewerage District for 18 years, including serving as its Chair during 1991, 1992, 2003, and 2004. He has also served on and chaired boards for nonprofit organizations across Wisconsin.

He enjoys bicycling across Wisconsin, hiking, backpacking and climbing throughout the United States, as well as building and maintaining hiking trails and doing other service work in wilderness areas with the Sierra Club and Wilderness Volunteers.