Janvrin entered the Royal Navy in 1964, was commissioned as an acting sub-lieutenant on 1 September 1966, promoted lieutenant on 4 March 1971, and served until 2 July 1975. He subsequently became a member of the Castaways' Club. On leaving the navy, Janvrin joined the Foreign and Commonwealth Office. He was a Second Secretary in 1975 and was appointed First Secretary at the mission to NATO in 1976. He was officially appointed an Officer of the Diplomatic Service on 7 February 1979. Janvrin was First Secretary in New Delhi from 1981 to 1984, during which time he was made a Member of the 4th Class of the Royal Victorian Order for services during the Queen's state visit to India.