[2] In 1999, he was also the EU's official candidate for leading the United Nations Development Programme (UNDP); the post instead went to Mark Malloch Brown.

[3][4] On 17 September 1999 Nielson became European Commissioner for Development and Humanitarian Aid when the Prodi Commission took office.

[5] Early in his tenure, the EU signed the Cotonou Agreement with its 71 partners from the African, Caribbean and Pacific Group of States (ACP) which put relations between Europe and its former colonies on a new footing for the following 20 years.

[6] In 2002, Nielson and Margot Wallström led the European Commission's team at the World Summit on Sustainable Development.

[8] In April 2015, Nielson was asked to conduct a strategic review of the labour market on behalf of the Nordic Council.