He was born in Verdun, now part of Montreal, to a working class family. After graduating from the Université de Montréal, he studied at the University of Paris where he met Pierre Trudeau, with whom he cofounded the dissident political magazine Cité Libre upon returning to Montreal.
He was the Jeunesse Étudiante Chrétienne (JEC)Canadian representative at the International Young Catholic Students(IYCS ) Centre for International Documentation and Information (CIDI) in 1947-49. He is considered as one of the key man behind the creation of IYCS which today is present in over 80 countries with millions of members.
Juneau joined the National Film Board of Canada in 1949. In the 1950s, he was the NFB's assistant regional supervisor in Quebec, and then became the chief of international distribution, the assistant head of the European office, and the NFB's secretary. In 1964, he became the board's Director of French-language production. In 1959, Juneau was a cofounder of the Montreal International Film Festival, and served as its president until 1968.
In 1966, Juneau was appointed vice-chairman of the Bureau of Broadcast Governors. When the BBG became the Canadian Radio and Television Commission (CRTC) in 1968, Juneau became the body's first chairman. In the early 1970s, he was the architect of the CRTC's Canadian content regulations that require a certain percentage of radio and television time to be devoted to programming (or music in the case of radio) produced in Canada. Canadian content, by requiring radio stations to give air play to Canadian artists, is credited with the creating a domestic market for Canadian music and the subsequent boom in music production. The music industry's Juno Awards are named after Juneau, and in 1971 he received a special Juno award for "Canadian music industry Man of the Year".
In 1975, Juneau left the CRTC to accept an appointment by Prime Minister Pierre Trudeau to the cabinet as Minister of Communications. Since Juneau did not have a seat in the Canadian House of Commons, he attempted to enter parliament through a by-election, but was defeated in the Montreal riding of Hochelaga by the Progressive Conservative candidate. He resigned from cabinet.
He was subsequently appointed to the civil service by Trudeau as undersecretary of state, and then, in 1980, as deputy minister of communications. In 1982, he became president of the Canadian Broadcasting Corporation. As Juneau was closely identified with the Liberal Party, he was viewed with hostility by the Progressive Conservative government of Brian Mulroney that came to power in the 1984 election.
Despite financial pressures, during his term as CBC president, Juneau inaugurated a new cable service, CBC Newsworld, and increased Canadian content on the CBC to 95% of programming.
After retiring from the CBC, he founded the World Radio and Television Council, a non-government organization supported by UNESCO.