Reform, led by the veteran campaigner for Brexit, hopes Thursday's local elections in England will mark the breakdown of a century of domination of politics in the United Kingdom by the governing Labour Party and opposition Conservatives.
In the most closely watched contest, for the vacant parliamentary seat of Runcorn and Helsby, Reform won by just six votes after a full recount, giving Farage's party its fifth seat in the House of Commons.
Labour had won the seat in last year's general election with a majority of almost 15,000.
"It's been a huge night for Reform," Farage told reporters.
"This is heartland Labour Party, their vote has collapsed and much of it has come to us."
The Conservatives - historically one of the most successful parties of any modern democracy - were now "toast," he said.
"You're witnessing the end of a party that's been around since 1832."
Andrea Jenkyns, a former Conservative minister who defected to Reform after losing her seat last year, became mayor of Greater Lincolnshire, which covers about a million people, making her the party's most powerful elected politician yet.
Reform won another mayoralty in Hull and East Yorkshire.
Reform also took control of several county councils including Kent, Derbyshire, Durham and Nottinghamshire.
It was leading all parties in the initial tally of councillors elected, with well over half of the more than 1600 seats up for grabs on local authorities declared.
Farage has staked his anti-establishment and anti-immigration party's claim to represent the right of UK politics since the Conservatives lost more than half of their support in a general election last year.
Labour Prime Minister Keir Starmer, who won one of the largest parliamentary majorities in UK history last northern summer, has since suffered the fastest decline in popularity of any newly elected government.
His government has raised taxes and cut benefits for the elderly, saying the moves were necessary to restore financial stability after 14 years of Conservative mismanagement.
Starmer said the results were "disappointing" but said he and his party colleagues "get it", adding: "I'm determined we will go further and faster on the change people want to see."
While still focused on immigration, Reform has begun offering solutions to the UK's stubborn problems such as an ailing health service and poor economic growth.
"Reform UK looks like the real deal this morning," said Keiran Pedley, Director of Politics at polling firm Ipsos.
"That said, with increased support comes scrutiny. Where they do win, they will need to show they can deliver the change their voters want."
Voters cast their ballots for local authority seats across much of England and six high-profile mayoral elections, with results coming in throughout Friday.
Runcorn and Helsby was the only parliamentary seat included after its Labour MP quit following a conviction for punching one of his constituents.
A Labour spokesperson said the results showed that voters "clearly expect the government to move faster" to change the UK after 14 years of Conservative rule.
Reform, founded as the Brexit Party in 2018, was initially written off as a narrow anti-immigration party.
But it has seen a surge in its support over recent months.
The party hopes the results will help make it the best positioned to challenge Labour and the Conservatives at the next general election, expected in 2029.
