He served as the Special Representative of the Secretary-General of the United Nations for Timor-Leste and head of peacekeeping and peacebuilding missions, UNMISET, UNOTIL and UNMIT from May 2004 to September 2006.

[1] He is currently the President of the Global Peacebuidling Association of Japan,[2] the ACUNS (Academic Council on the United Nations System) Liaison Officer in Tokyo,[3] the Chair of the Hiroshima Peacebuilders Center (HPC) Council[4] and the Personal Advisor to former president and Prime Minister José Ramos-Horta of Timor-Leste.

[1] Hasegawa entered the United Nations Development Programme (UNDP) in 1969, and then spent 37 years of his professional career as an international civil servant.

[5] From April 2007 to March 2016, Hasegawa had been teaching at Hosei University on peace-building, international organizations, contemporary foreign policy and global governance.

He has been an invited as a speaker and a senior mentor at many meetings, conferences and seminars including those held in Kuala Lumpur, Malaysia in 2009, Cairo, Egypt in 2009 and 2010, Yaoundé, Cameroon in 2012, Hiroshima and Tokyo, Japan in 2007–2013 on peacekeeping and peacebuilding operations.