[3] Soon after graduation, Mecacci started to work in New York for No Peace Without Justice and the Transnational Radical Party (TRP), a non-governmental organization with the consultative status at the Economic and Social Council (ECOSOC) of the United Nations (UN).

As a MoP, in January 2009 he came to the fore for organizing parliamentary obstruction against the approval of the Friendship Treaty[6] between Italy and Libya signed by Silvio Berlusconi and colonel Muammar Gaddafi.

On 9 November 2010, with 274 yes and 261 no, a motion[9] presented by him was approved in the Chamber, despite the contrary opinion of the Italian government and the majority parliamentary groups, precisely on the Italy-Libya friendship treaty, which provides for the commitment to clarify the terms of the agreements.

From August 2013 until October 2013 Mecacci led the OSCE/ODIHR Election Observation Mission for the Presidential Election in Georgia In December 2013, Mecacci was selected as President of the International Campaign for Tibet, a leading international human rights groups advocating for the respect of human rights and Democratic freedom for the Tibetan Peale and for a negotiated solution with the Chinese government.

In May 2011 he gave a lecture on the role of the OSCE in the master program of Peacekeeping and Security Studies at the Faculty of Political Science - Roma Tre University.

Over the period of 2010–2020, Mecacci attended numerous governmental, parliamentary and non-governmental conferences in different parts of the world[citation needed], on issues related to the promotion of democracy and the protection of human rights, both in the United Nations and in the European Union.