While at university, Grimes was an activist for the Liberal Democrats, and worked for then-MP Norman Lamb's unsuccessful 2015 party leadership campaign. The following year he founded the pro-Brexit group BeLeave aimed at younger voters during the 2016 United Kingdom European Union membership referendum campaign. Grimes later dropped out of university, and between 2016 and 2018, he worked as a deputy editor for the political website BrexitCentral, founded by Matthew Elliott, the former Vote Leave chief executive. In 2018, he became the digital manager for the Institute of Economic Affairs, a right-wing think tank. In 2019, together with commentators Steven Edgington, Chloe Westley, Dominique Samuels, Tom Harwood and others, he was amongst those associated with a newly launched right-wing youth organisation called Turning Point UK. The organisation was set-up by Tory-party donor and unsuccessful MEP candidate for the Brexit PartyGeorge Farmer, but the organisation refused to disclose its other donors. The project was endorsed by - among others - Priti Patel, Jacob Rees-Mogg and Nigel Farage. It is closely allied to Turning Point USA, a controversial pro-Trump youth movement. TPUK caused outrage when it encouraged students to video any "perceived left-wing bias" of university lecturers. In May 2020, he launched ReasonedUK, a platform for those "standing against the tide" who "hide political views for fear of being called homophobic, a TERF, racist". Some criticised perceived transphobia voiced by Grimes during the launch. The magazine Private Eye revealed that the platform ReasonedUK is produced by the son of former Brexit Party MEP Lance Forman. A video has also been released by Grimes on ReasonedUK that appears to be a near word-for-word copy of a video released by the US right-wing platform PragerU. In July 2020, an interview with the historian David Starkey that Grimes published on his video platform sparked significant controversy. The historian remarked that "Slavery was not genocide, otherwise there wouldn't be so many damn blacks in Africa or in Britain, would there?" This prompted backlash, including condemnation by former Chancellor of the ExchequerSajid Javid. Starkey subsequently resigned from his fellowship at Fitzwilliam College, Cambridge, was fired by Canterbury Christ Church University, and was dropped by current and former publishers. Following the airing of his interview with Starkey, Grimes blamed his inexperience at interviewing for his failure to pick up on and challenge Starkey's comments. Shortly after, Grimes gained criticism again for inviting the controversial historian Andrew Roberts on to his show, who has been associated with pro-Apartheid movements and criticised for defending the Amritsar massacre.
In 2018, Grimes was fined £20,000 by the Electoral Commission after it determined that there was evidence that BeLeave had spent more than £675,000 with the Canadian political consultancy firmAggregateIQ in coordination with the official Brexit campaign organisation Vote Leave in distribution targeted social media advertisements. The Commission argued that these actions violated electoral spending rules, and that Grimes and Vote Leave official David Alan Halsall had made false declarations relating to the spending. Vote Leave's fine was upheld on appeal, but Grimes' was overturned. Subsequently, in May 2020, the Metropolitan Police ended its investigation into Grimes and Halsall.
