Lyne was a member of the Diplomatic Service from 1970 to 2004. During the earlier part of his career he served in the Soviet Union, Senegal and at the the United Nations in New York, as well as in the Soviet and Rhodesian departments of the Foreign Office and as Assistant Private Secretary to the Foreign Secretary.
From 1993 to 1996 he served as Private Secretary to the Prime Minister for foreign affairs, defence and Northern Ireland. From 1997 to 2000, he was UK Permanent Representative to the UN, the World Trade Organisation, and other international bodies in Geneva. From 2000 to 2004, he was the British Ambassador to Moscow.
- Royal Institute of International Affairs - Deputy Chairman
- Ditchley Foundation - Governor
- Kingston University - Board of Governors
- Peter Hambro Mining - Non-executive director
- Iraq Inquiry - Member