Ambassador Sauer began his diplomatic career with the Finnish Foreign Ministry in 1995 working as a Desk Officer for Germany, Austria, Switzerland, United States and Canada in the Political Department. In 1997, Sauer transferred to the Economic Department to work as a Desk Officer for Cooperation with Adjacent Areas. In 1998, he worked for the Department for Press and Culture as the Desk Officer for Cultural Cooperation.

Sauer served as the Deputy Head of the Embassy of Finland in Zagreb, Croatia, as the First Secretary of the Permanent Mission of Finland to the United Nations and as a Senior Adviser to the Special Representative of the Secretary General, Mr. Harri Holkeri in the United Nations Interim Administration Mission in Kosovo (UNMIK). In 2004 Sauer worked as the Head of the Strategic Analysis and Research Unit in UNMIK.

In 2005, Sauer was appointed as the Director of the Unit for the Western Balkans, in the Ministry for Foreign Affairs of Finland and worked as a Senior Adviser to the Special Representative of the UN Secretary General, former President of the Republic of Finland, Martti Ahtisaari who led the UN-facilitated Kosovo future status process and the UN Office of the Special Envoy for Kosovo (UNOSEK) in Vienna, Austria.

In 2007 Sauer was appointed Director of the Unit for UN Affairs in the Ministry for Foreign Affairs. In 2010 Sauer was appointed as the Ambassador of Finland to Indonesia, Timor-Leste and ASEAN (Association of Southeast Asian Nations).

In 2014 Sauer served as the Permanent Representative of Finland to the United Nations where he served until 2019.