He was a member of ADR from 2008 to 2017. He joined the party on July 14, 2008 at the age of 16 as well as its youth organization in 2009. After five years of vice-presidency, he got elected on March 16, 2015 in Luxembourg City as chairman of the youth organization adrenalin. He was also the vice-president of the ADR South region and local section of Pétange, represented in the national and executive board of the party. In the local elections held on October 9, 2011 he ran at the age of 20 for a seat in the council of Pétange obtaining 403 voices and the first and only seat for ADR, enabling the party's first local success in the municipality. At the general meeting of the Pétange section, he got designated as head of the list for the upcoming local elections on October 8, 2017. In the Luxembourg General election held on October 20, 2013 he ran for a seat in the Chamber of Deputiesfor the south, obtaining 5234 voices placing him fourth on the list. At the congress of the European Young Conservatives in December, 2016 in Brussels, he got elected 1. vice-chairman of the board of the European, conservative youth-organizations EYC. In April 2012, Thein was given a ban on speaking in the lyceum, after students invited the politician, as speaker on the subject of multiculturalism. This led to criticism from the students and his party in the press. Thein himself spoke of a lack of political neutrality of the school administration and boycott. In April 2014, Thein initiated a petition against the right of foreigners to vote, which was submitted and approved by the Petition Commission of the Chambre des Députés. The aim was, according to Thein, the promotion of Luxembourg's sovereignty and identity, as well as the integration policy in Luxembourg. On March 9, 2017 after a disciplinary procedure, he was expelled from his party due to a controversial "Like" on Facebook. Prior to the measure, the justice proclaimed that no prosecution will be initiated because of no evidence of hatespeech. Yet, the party board voted for his exclusion. Soon after, he announced the possible founding of a "real conservative party."
On March 21, 2017 he founded a new political party in Luxembourg called déi Konservativ, of which he is the current chairman. On April 10, he got elected in his political office at the extraordinary national congress of the party, and re-elected with all voices at the annual national congress on January 7, 2018. In the local elections held on October 9, 2017 his party ran with a list in the municipality of Pétange, with Thein as head of list, obtaining 2,40 % as a party result, with a personal vote of 550 voices. On January 29, 2018 he was elected by the municipality council into the commission of integration for the political term 2017/2023. In the Luxembourg General election held on October 14, 2018 his party ran with a list in the south, with Thein as head of list, obtaining 0,52 % as party result, with a personal vote of 1.426 voices placing him first on the list.
Other
In 2016, Thein was portrayed in a short documentary of the documentary series routwäissgro of the film production Calach Films and Kollektiv 13, as well as with the support of the Film Fund Luxembourg and RTL Télé Lëtzebuerg, entitled “Meng Heemecht” and shown on national television RTL. On February 4, 2018 a political assassination of Thein was attempted. A strong firecracker blew up several windows of Thein's house and party headquarters. A car was also damaged. The police initiated a criminal investigation into the case. In October 2018, Thein initiated police report against musician Tun Tonnar, subsequently followed by a process in March 2019. In a songtext, entitled "FCK LXB", Thein as well as his party had been mentioned in the lyrics, along with a row of insults against Luxembourg, the Grand-Duke of Luxembourg, luxembourgish institutions and other parties and politicians. In the process on the district court of Luxembourg on March 28, Thein demanded 5.000 € as damages because of public injury of reputation and insult. A judgement is fixed for May, 8.
