After retirement, he was appointed as Special Senior Adviser to the President of the UN General Assembly.
[1][2] Sen presented his credentials as Permanent Representative of India to the United Nations on 17 September 2004.
Prior to this appointment, Sen had been India's High Commissioner to Sri Lanka.
He has previously headed Indian missions in Sofia and Oslo in Bulgaria and Norway.
His predecessor as Permanent Representative of India to the United Nations was Vijay K. Nambiar his successor is Hardeep Puri.