It is likely that Klimaitis was an officer in the Lithuanian Army. During the pre-war years he was editor of the tabloid Dešimt centų (Ten Cents). His attitudes shifted to anti-communism and anti-semitism. He joined the Voldemarininkai movement.
When the Nazi Germans occupied Lithuania in June 1941, at the start of Operation Barbarossa, Klimaitis formed a military unit of roughly 600 members, which was not subordinate to the Lithuanian Activist Front or the Provisional Government of Lithuania, and engaged in firefights with the Soviet army for the control of Kaunas. On the evening of June 23, most of the city was in the hands of the insurgents.
On the night of 25–26 June, Kaunas pogrom led by Klimaitis' unit was instigated by SS Brigadeführer Franz Walter Stahlecker, commanding officer of Einsatzgruppe A. By 28 June 1941, according to Stahlecker, 3,800 people had been killed in Kaunas and a further 1,200 in surrounding towns in the region. Klimaitis' men destroyed several synagogues and about sixty Jewish houses. Modern sources claim that the number of victims in Stahlecker's report were probably exaggerated.
After the war, Klimaitis moved to Hamburg, Germany, where he was discovered in the late 1970s. Hamburg Police launched an investigation, but Klimaitis died before the case could be brought to trial. He died in 1988.
- In non-Lithuanian texts he is sometimes incorrectly referred to as Jonas Klimaitis. Stahlecker's report did not supply his first name.
- Baltic News
- Voren, Robert van (2011). Undigested Past: The Holocaust in Lithuania. On the Boundary of Two Worlds: Identity, Freedom, and Moral Imagination in the Baltics. Rodopi. p. 78. ISBN 9789401200707.
- Bubnys, Arūnas (1997). "Vokiečių ir lietuvių saugumo policija (1941–1944) (German and Lithuanian security police: 1941–1944)". Genocidas ir rezistencija (in Lithuanian). I. ISSN 1392-3463. Retrieved 2006-06-09.
- Gitelman, Zvi (1998). Bitter Legacy: Confronting the Holocaust in the USSR. Indiana University Press. pp. 97, 102. ISBN 0-253-33359-8.
- "The Einsatzgruppen: Report by Einsatzgruppe A in the Baltic Countries (October 15, 1941)". Jewish Virtual Library. American-Israeli Cooperative Enterprise. Retrieved 2008-08-06.
- Milkeraitis, Kęstutis (2007-12-05). "Vestuvių generolai ir Lietuvos saugumas". XXI amžius (in Lithuanian). 90 (1587). ISSN 2029-1299.