Between 2001–03, Hammarberg acted as Regional Adviser for Europe, Central Asia and the Caucasus for the UN High Commissioner for Human Rights.

For several years, he was the Swedish Prime Minister's Personal Representative for the UN Special Session on Children, as well as the Convener of the Aspen Institute Roundtables on "Human Rights in Peace Missions".

As Commissioner, Hammarberg regularly conducted visits to promote the respect of human rights in all Council of Europe member states, in accordance with his mandate.

In a number of speeches and statements, Hammarberg actively seeks to improve living conditions for the largest minority in Europe and criticises the alarming levels of racism directed at these people.

Hammarberg paints a clear picture of the situation; for example, in his latest report on Italy he heavily criticises the Italian authorities over their treatment of Sinti and Roma people.

On 27 February 2012, he published a comprehensive report on the situation of Roma and travellers in Europe,[5] stressing that in many European countries they are still denied basic human rights and suffer blatant racism.

He repeated this “urgent recommendation” one year later in a letter to the then German Federal Minister of the Interior, Thomas de Mazière.