He joined Christian Democratic People's Party in 1991 and served as leader of the party's branch in Kispest from 1994 to 1996. After the party's decline and disintegration, he left KDNP at the end of 1996 and joined the strengthening right-wing Fidesz party. Simicskó was elected lawmaker from the Budapest regional list of the Fidesz in the 1998 parliamentary election. He became Vice-Chairman of the Committee of National Defence, a position which he had held until 2006 with short interrupts. After Hungary joining the NATO in 1999, he was delegated to the Defence and Security Committee of the organization's Parliamentary Assembly. He served as Political State Secretary for Ervin Demeter, the Minister of Civilian Intelligence Services, from 3 May 2000 to 27 May 2002, during the First Orbán Cabinet. In the 2002 national election, Simicskó was re-elected parliamentarian, however the Fidesz was defeated by the MSZP–SZDSZ coalition. In April 2003, Simicskó was the only MP who voted against joining the European Union following a successful membership referendum, where the proposal was approved by 83.8% of voters with a voter turnout of 45.6%. Simicskó then argued that there were numerous "questions and uncertainty" around the enlargement and also added, he represented the "majority of the non-voters" when he rejected the parliament's authorization. Simicskó rejoined the Christian Democrats, which re-established after continuous inward struggles and legal disputes and entered into an alliance with Fidesz. He was elected one of the vice presidents of the party in 2006. In the 2006 parliamentary election, he became Member of Parliament again and now joined the KDNP parliamentary group. From 2006 to 2010, he functioned as Chairman of the National Security Committee. Simicskó was elected MP for Újbuda during the 2010 parliamentary election. Following the landslide victory of the Fidesz in the 2010 parliamentary election, Simicskó was appointed Secretary of State for Defence on 2 June 2010, under Minister Csaba Hende. According to press reports, the relationship between Hende and Simicskó was tense and conflictual in the next two years. On 7 October 2012, Simicskó was replaced as Secretary of State for Defence by MP Tamás Vargha. He became Secretary of State for Sports in the Ministry of Human Resources on the following day, succeeding Attila Czene. In the 2014 parliamentary election Simicskó was re-elected MP again, representing Újbuda. Simicskó was appointed to the position of Minister of Defence in September 2015 after the resignation of Csaba Hende during the European migrant crisis, with the speed at constructing a wall on the border with Serbia being an issue for Prime MinisterViktor Orbán. Simicskó was re-elected MP for Újbuda during the 2018 parliamentary election. He was replaced as Minister of Defence by Col. Gen. Tibor Benkő. Simicskó was appointed one of the recorders of the National Assembly in June 2018. He held the latter position until July 2020, when he was elected leader of the KDNP parliamentary group, replacing Péter Harrach.