He joined the Department of External Affairs on graduating and held diplomatic posts in India, Fiji and Saudi Arabia.

John Knight represented the Commonwealth government at the bicentenary celebrations of Captain James Cook's arrival in Hawaii, in January 1978.

He was later awarded a grant by the East–West Center to study at the University of Hawaii, completing his Master of Arts thesis on Pakistani leader Muhammad Ali Jinnah.

He took leave without pay in 1973 to join the office of Liberal opposition leader Billy Snedden as senior private secretary, where he was an influential adviser on foreign policy.

[1] Knight left Snedden's office in 1974 to take up a three-month visiting fellowship at the Research School of Pacific Studies at the Australian National University.

He then rejoined the Department of Foreign Affairs, initially working in its United Nations section and then in August 1975 being posted as a counsellor at the Australian embassy in Jeddah, Saudi Arabia.

[1] In November 1975, following the Whitlam dismissal, Knight won Liberal preselection as the party's lead Senate candidate for the newly created Australian Capital Territory seats.