Galvin went on to spend the rest of his career with the National Park Service, in the U.S. Department of the Interior—serving in various national parks and NPS regional offices, training centers, service centers, and the headquarters office in Washington. Over his 38-year NPS career, he worked as an engineer at Mount Rainier National Park, and in the Service's Southeast Regional Office in Atlanta; as a training specialist at the agency's Horace M. Albright Training Center in Grand Canyon; and as a management assistant at the New York District Office, overseeing park operations for a variety of NPS facilities in the New York City area. In 1974, he was appointed Associate Regional Director for Operations for the NPS Northeast Regional Office in Boston, and in 1976 became Deputy Director for that Region. Subsequently, for seven years beginning in 1978, he headed up the Service's large Denver Service Center, in Denver, Colorado, responsible for overseeing Park Planning, Design, Development and Construction in the National Park System nationwide. In 1985, Galvin was appointed to the Service's second highest position as Deputy Director, and served in that and several other posts over the rest of his career in the NPS Headquarters office in Washington. He served as Deputy Director for a combined total of nine years, spanning the Reagan, Clinton, and George W. Bush Administrations. During most of his tenure as Deputy Director, Galvin was the highest ranking career government official in the Service. As such, he was called upon to officially represent the Service in more than 200 formal Congressional hearings before the United States Congress on Capitol Hill. In his capacity as the Service's Deputy Director, he was called upon numerous times to serve as Acting Director, on occasion, for extended periods of time.
Galvin retired from the National Park Service in 2002. Over his long NPS career, he had come to be widely respected and admired, both within and outside the agency, for his contributions to the America's National Parks. Then NPS Director William Penn Mott Jr. had said of him in 1985, when announcing Galvin's appointment as Deputy Director: "He is... an effective, decisive leader whose work has drawn praise from both park professionals and interested observers outside of government." U.S. Congressman Nick J. Rahall II, upon the occasion of Galvin's retirement in 2002, said: "For many people, both within and outside of the National Park Service, Deny is 'Mr. Park Service'.... It is heartening to see a civil servant who has exhibited such a love for his work." Following his retirement, Galvin has continued to work in support of parks and conservation, and has on occasion still been asked to testify in Congress. He has also frequently been sought out by the media to comment on Park Service matters. Writing for The George Wright Forum in 2007, in reference to the 1916 National Park Service Organic Act's mandate to "promote" Parks, Galvin offered an expanded vision for the 21st Century: "There remains a need to promote the parks, not to bring people to them, but to promulgate the values they have come to represent." In 2008–2009, he served as a member of the National Parks Conservation Association's Second Century Commission for the National Parks, co-chaired by former U.S. Senators Howard Baker and Bennett Johnston. He served as a consultant for the Ken Burns 2009 television documentary film, . He is also a contributor to the work of the Library of American Landscape History; serves as an adviser to the Coalition of National Park Service retirees; and is on the Board of Directors of the National Parks Conservation Association,
