Jonathan was Chief of Staff to Tony Blair from 1997 to 2007 and the chief British government negotiator on Northern Ireland during that time in office.

Jonathan was a British diplomat from 1979 to 1996 working on the negotiations to return Hong Kong to China in the early 1980s, the CSCE human rights talks, CDE arms control talks with the Soviet Union in the mid-1980s, and the ‘Two plus Four’’ talks on German reunification in the late 1980s.

As the British Government's chief negotiator on Northern Ireland, he played a key part in leading the peace negotiations to a successful conclusion, from the Good Friday Agreement in 1998 through the nine year battle to get a lasting settlement in place. He subsequently published "Great Hatred Little Room: Making Peace in Northern Ireland" giving a personal account of those painstaking negotiations and “Talking to Terrorists”. Jonathan has also participated in a number of negotiations between governments and insurgent groups in Europe and Asia working closely with Martin Griffiths at the Centre for Humanitarian Dialogue.

The organisation they created, Inter Mediate, focuses on the most difficult conflicts where other organisations are unable to operate. It brings together leading experts on dialogue and negotiation, who operate as a flexible team that fills a vital gap in the conflict resolution landscape. Through the facilitation of negotiation, Inter Mediate hopes to contribute towards a sustainable end to conflict.