Lazzarini started his international career in 1989 when he joined the International Committee of the Red Cross (ICRC) and served as delegate in multiple duty stations, including Southern Sudan, Lebanon, Jordan, and Gaza. Lazzarini then led the ICRC operations in Bosnia, Angola, and Rwanda. He served as Deputy Head of the ICRC Communications Department. In September 1999, Lazzarini joined the Union Bancaire Privée bank in Geneva as head of the Marketing Department where he worked on corporate social responsibility and the bank's ethical investment agenda.
In 2003, Lazzarini joined the UN Office for the Coordination of Humanitarian Affairs (OCHA) as Area Coordinator in Mossul, Iraq. He then served as Head of Office for OCHA in Angola, Somalia, and the occupied Palestinian territory. In 2010, he became the Deputy Director of the Coordination and Response Division for OCHA in its headquarters in New York City. In 2013, he was appointed,[2] under the leadership of Nicholas Kay, as the Deputy Special Representative of the Secretary-General, UN Resident and Humanitarian Coordinator and Resident Representative of the United Nations Development Programme (UNDP) in Somalia. On 24 April 2015, United Nations Secretary-GeneralBan Ki-moon announced[3] the Lazzarini's appointment as his Deputy Special Coordinator for Lebanon, where he also served as Resident and Humanitarian Coordinator and UNDP Resident Representative, succeeding Ross Mountain; in this capacity, he worked under the leadership of successive Special Coordinators Sigrid Kaag (2015-2017), Pernille Dahler Kardel (2017-2019) and Ján Kubiš (2019-2020).
In 2023, Lazzarini criticized the Israeli blockade of the Gaza Strip.[5][6] He described the Gaza Strip as a "graveyard of a population trapped between war, siege and deprivation", saying that "we will not be able to say we did not know. History will ask why the world did not have the courage to act decisively and stop this hell on Earth."[7] On 9 February 2024, Lazzarini said that Israel blocked food for 1.1 million Palestinians in Gaza.[8]
UNRWA, Lazzarini and other top UNRWA officials are co-defendants in a high-profile lawsuit by the October 7 attack victims alleging that "the defendants are liable for aiding and abetting Hamas’ genocide, crimes against humanity, and torture."[9][10]