After life at Read, Sir Leonard gained an honours degree in Classical Chinese from Queens College, Cambridge, which complemented his selection of languages including Mandarin, Russian, Hungarian and French.

His first posting was for the British Embassy in the People's Republic of China between 1966 and 1968. Following this, Sir Leonard served as First Secretary in the British High Commission in India between 1971–1974 and later returned to China from 1994 until 1997 as ambassador. He continued his ambassador services as the UK's ambassador to Hungary, in the Treasury, in the Cabinet Office as Deputy Cabinet Secretary, as Secretary of the Gulf War Cabinet, and as Financial Counsellor in Paris.

Following his departure from the his long standing, successful diplomatic service, Appleyard took up a position as vice-chairman of Barclays Capital and served as joint-Chairman on the Nuffield Languages Programme Steering Group along with Sir Trevor McDonald

He died on 7 February 2020 at the age of 81