Educated at Gresham's School, Holt, Hetherington became a student at Corpus Christi College, Oxford, in 1938.
In 1942 Hetherington was commissioned in the Royal Army Corps. He took part in the Normandy landings as a tank corps major. On behalf of the British control commission, he set up a quality newspaper, Die Welt, in Hamburg for the British zone in 1945. His position was editorial controller and he remained in this position until his discharge in autumn 1946.
In 1946 he joined the Glasgow Herald. He moved to the Guardian in 1950 as a leader writer and defence correspondent. He was foreign editor and assistant editor (and continued as defence correspondent) from 1953 to 1956. He became editor in 1956, just days before the Suez crisis. Under his editorship the Guardian dropped 'Manchester' from its title in 1959, started printing in London in 1961, and the editorial offices moved to London in 1964. The turbulent 1960s saw Hetherington fight off financial crises, including a proposed merger with The Times.
Hetherington retired from editorship in 1975, but retained his position as a Scott Trust trustee, 1970-1989. He was chair of the Scott Trust, 1984-1989.
He was controller of BBC Scotland, 1975-1978, and manager of BBC Highland, 1979-1980.
In c1982 Alastair and wife Sheila set up an independent television production company, Scotquest Ltd. In at least some of the programmes that they produced, Hetherington combined his favourite things: media, Scotland and walking.
Hetherington was Emeritus Professor at the University of Stirling from 1985-1989.