As academic sociologist

Himelfarb started his career as a professor of sociology at the University of New Brunswick in 1972. He stayed at UNB until 1981. With C. James Richardson, Himelfarb wrote two introductory textbooks on sociology, which were used extensively in Canadian universities in the late 1970s and early 1980s:

1. People, Power and Process (and a reader)
2. Sociology for Canadians (two editions, and a reader)

Himelfarb has published numerous monographs, chapters and articles on Canadian society and public policy and co-edited with his son Jordan the book Tax is Not a Four-Letter Word.

As civil servant

Himelfarb joined the Canadian public service in 1981 in the Department of the Solicitor General of Canada and served in senior positions in various departments and agencies, including the Parole Board of Canada, Justice, Citizenship and Immigration, the Privy Council Office, and Treasury Board, and led the Task Force on the Social Union. In 1999, he became deputy minister of Canadian heritage. In 2002 under Jean Chretien he was appointed to the dual role of clerk of the privy council and secretary to the cabinet.

On June 14, 2006, under Stephen Harper, Himelfarb was appointed ambassador to Italy, with concurrent accreditation to the Republic of Albania and the Republic of San Marino, and as High Commissioner in the Republic of Malta, and as permanent representative to the Food and Agriculture Organization, the World Food Programme and the International Fund for Agricultural Development, in Rome. He retired as ambassador in 2009.

As university administrator

In September 2009, Himelfarb was appointed as director of the Glendon School of Public and International Affairs, at York University, retiring from that position in 2014, when he was made director emeritus.