Bennett joined UKIP in 2014. He campaigned in the 2014 European election, and was agent for UKIP candidate Brian Morris in Cardiff West at the 2015 general election. When Bennett was selected to contest the South Wales Central electoral region, he was criticised for blaming rubbish problems in Cardiff on students and Eastern European immigrants and was nicknamed the 'Donald Trump of Wales'. This led to senior UKIP figures including leader Nigel Farage and immigration spokesman Steven Woolfe publicly distancing themselves from Bennett. A petition containing the names of 16 other UKIP Assembly candidates opposing Bennett's candidacy was presented to the party's National Executive Committee, which upheld his candidacy. He was elected to National Assembly for Wales as member for South Wales Central in 2016. In the UKIP Assembly group, Bennett backed Neil Hamilton for leader over Nathan Gill. After Hamilton became leader of the group, Bennett was appointed as UKIP Assembly spokesman for Local Government, Housing and Sport. In April 2017, Bennett was appointed as UKIP's Business Manager in the Assembly. In the same month, the Assembly's Equalities Committee, of which Bennett was a member, published a report about refugees. Bennett brought out his own report, in which he complained that the proposals would encourage more refugees to come to Wales. In December 2017, Bennett was banned from speaking in the Chamber of the Welsh Assembly for a year by Presiding Officer Elin Jones. The ban came after Bennett claimed that making the process to change legal gender easier would lead society to implode. Bennett refused to apologise or withdraw his remarks, and was barred. In January 2018, Bennett made a partial apology in which he stated that he would accept the authority of the Presiding Officer, and he was allowed to resume his speaking duties without withdrawing his previous remarks. In February 2018, Bennett led UKIP's opposition to plans to expand the Welsh Assembly's membership. Bennett claimed almost £10,000 in expenses to set up an office which never opened. In July 2018, it was announced that there would be a ballot of UKIP party members in Wales to decide the position of UKIP Leader in Wales. The three candidates who announced that they were running for the post were former Assembly Group Leader Neil Hamilton, who had just been ousted from the role; Caroline Jones, who had ousted him; and Gareth Bennett. Bennett's main policy planks included his campaign to abolish the Welsh Assembly, and his opposition to the Welsh Government's Welsh language policies. He won the UKIP Wales leadership election in August 2018, with 269 votes. In August 2018, Bennett was criticised by First Minister Carwyn Jones and other Assembly members for supporting Boris Johnson's remarks about the Muslim veil. On 3 June 2019, Bennett announced that he would be running in the 2019 UK Independence Party leadership election after Gerard Batten stood down as leader. Bennett has claimed that he would take UKIP forward with 'bold policy ideas', after Gerard Batten's one-year term came to an end. Bennett published his 2019 UKIP Leadership Election Manifesto on his website, which includes policies such as 'National ID Cards','UKIP Party Democracy', 'National Volunteer Force', 'Regular Referenda', 'Freedom of Speech', 'Regulating Mosques', 'Scrap Foreign Aid' and 'Abolishing Politicians'. On 11 July, Bennett announced that he would withdraw his leadership bid in order to promote Ben Walker's campaign. On 7 November 2019, he quit UKIP to become an independent politician.
