Strong began a business career as a trading-post employee for the Hudson's Bay Company in the Arctic in 1944. His business talents developed as he quickly moved from being accountant for a mining group in Toronto (1945) through a series of management and investment positions with various energy and financial corporations (1948–66).
In 1966 he shifted to international and later environmental affairs. He headed the Canada International Development Assistance Programme and its successor, the Canadian International Development Agency, until 1970, defining its long-term strategies. He then was secretary-general of the United Nations Conference on the Human Environment and undersecretary-general of the UN Switzerland office (1970–72), then executive director of the UN Environment Programme in Nairobi, Kenya (1973–75).
Strong returned to business as head of Petro-Canada (1976–78), after which he became chairman of the International Energy Development Corp (1980–83), chairman of the Canada Development Investment Corp (1982–84), and again undersecretary-general of the UN (1985–87 and 1989–92). As secretary-general of the UN Conference on Environment and Development, Strong co-ordinated the historic conference on the environment held at Rio de Janiero, Brazil in June 1992. As chairman of Ontario Hydro (1992–95), he was credited with stabilizing its finances. In 1997 Strong was named senior advisor on reforming the United Nations.
His many volunteer activities included positions with the International Union for the Conservation of Nature and Natural Resources; the World Wildlife Fund; the World Council of Churches; the Society for Development, Justice, and Peace (the Vatican); the North-South Institute; and the World Commission on Environment and Development.
Strong's numerous conservation and humanitarian awards include the Freedom Festival Award (1975) and the first Pahlavi Environment Prize (1976). He became an Officer of the Order of Canada in 1976 and Companion of the Order in 1999, and received more than 25 honorary degrees.