A native of Vukovar, Merčep worked as an engineer before joining the Croatian Democratic Union (HDZ) in 1990. He then entered the local city government as the Secretary of People's Defense (Croatian: Sekretar narodne obrane), where he exerted considerable power in the local police and business, esp. in preparation for the impending war. During the war, he engaged in paramilitary activities which were subsequently investigated by the Hague tribunal and covered by the (now defunct) Croatian newspaper Feral Tribune.
At the turn of 1991, several properties owned by ethnic Serbs were blown up in Vukovar, and it was widely speculated that Merčep was behind this. In 1997 Feral Tribune released a document which confirmed exchanges of large quantities of explosive materials in September 1990 between Merčep and Branimir Glavaš. In August 1991, Merčep was briefly arrested by Croatian authorities and detained on undisclosed charges, but was soon released and moved to Zagreb together with his family, a week before the Battle of Vukovar started.
Merčep later became an officer in the Croatian Ministry of Internal Affairs and he participated in the other fronts of the Croatian War of Independence, being in command of thousands of paramilitaries which were responsible for killing and expelling thousands of ethnic Serbs from areas in and around Gospić, among other places (notably the Murder of the Zec family in Zagreb). A decade later, five members of his unit, Munib Suljić, Igor Mikola, Siniša Rimac, Miro Bajramović and Branko Šarić, were indicted on several criminal charges related to the so-called "Pakračka poljana" case, involving the killing of prisoners, mostly ethnic Serbs, in a field near Pakrac, and later convicted. Tomislav Merčep himself was not indicted in these proceedings.
Merčep became a HDZ member of the Chamber of Counties of Croatian Parliament in 1993.
In 1995, he became the leader of the "Association of Croatian Volunteer Veterans of the Patriotic War" (Croatian: Udruga hrvatskih dragovoljaca Domovinskog rata, UHDDR). As of 2010[update] he remains at the head of that association.
In late 1990s he quit the HDZ and instead founded his own party, the Croatian Popular Party (Hrvatska pučka stranka, HPS). In 2000 he ran as a HPS candidate on 2000 presidential elections, where he received 0.85% of the vote and was eliminated in the first round.
In 2003, the Croatian weekly Nacional reported that the ICTY was "completing an indictment against Tomislav Merčep", after having talked to Franjo Gregurić, Mladen Markač, Hrvoje Šarinić and others. There were media reports in 2006 that an indictment against Merčep himself, based on ICTY investigations, was forthcoming in the Croatian legal system. In December 2010, Amnesty International stated Merčep should be prosecuted based on a series of public testimonies about crimes committed by his subordinates. In the same week, the County Prosecutor in Zagreb brought up charges against Merčep and he was arrested.
- "IWPR Tribunal Update No. 545 - Six Croatians indicted for war crimes". Institute for War and Peace Reporting. 2008-04-08. Retrieved 2010-01-05. "The arrests are based on evidence originally gathered by the International Criminal Tribunal for the former Yugoslavia, ICTY, during its investigation of Tomislav Mercep, the commander of reservist police units."
- "Dossier: Pakračka Poljana". Yupress.com excerpts from Feral Tribune. May 19, 1995.
- Davor Butković (2005-09-17). "Pobjeda pravde". Jutarnji list (in Croatian). Retrieved 2010-01-05.
- "Vrhovni Sud Republike Hrvatske-Presuda i rješenje broj: I Kž 81/06-7" (in Croatian). Supreme Court of the Republic of Croatia. 2006-05-10. Retrieved 2010-01-05.
- Mladen Pleše (2003-03-12). "Merčep going to the Hague, Gregurić before investigators". Nacional. Archived from the original on 25 July 2012. Retrieved 25 July 2012.
- "Uskoro optužnica protiv Merčepa?". Jutarnji list/Nova TV (in Croatian). 2006-09-01.
- "Amnesty o ratnim zločinima: Zašto su Šeks, Merčep i Domazet nedodirljivi?". Jutarnji list (in Croatian). 2010-12-08. Retrieved 2010-12-09.
- "Croatia urged to speed up war crimes prosecutions". Amnesty International. 2010-12-09. Retrieved 2010-12-09.
- Lukić, Slavica (2010-12-10). "Zbog ratnog zločina iz 1991. uhićen Tomislav Merčep!". Jutarnji list (in Croatian). Retrieved 2010-12-10.
- Biography (Croatian)