Sacked by the Conservatives when his potential switch was leaked on Thursday, Jenrick appeared alongside Farage at a press conference, the latest of 21 current or former Conservative lawmakers to join Reform.
Farage's populist party is ahead of both Prime Minister Keir Starmer's Labour and Kemi Badenoch's Conservatives in polls before an election due in 2029 that could end the two-party system dominating for more than a century.
"The two main parties are rotten," Jenrick, 44, said.
"They are no longer fit for purpose. They both broke Britain, and neither can fix it."
Jenrick, who served in a number of senior government roles including immigration minister, was the second sitting Conservative MP to switch to Reform, giving it six seats in parliament.
Badenoch had earlier in the day announced Jenrick's sacking from her policy team and suspension from the party after she received evidence showing he was planning to defect.
He lost to her in the 2024 contest to lead the main opposition party after their crushing general election defeat and was then, in an effort to reunite the Conservatives, given the role of justice spokesman.
The former centrist, who is a qualified lawyer, used that position to build a personal profile on issues like immigration and crime that many saw as a platform for a future challenge to Badenoch's leadership.
Reform's popularity has come despite criticism from opponents that it lacks fleshed-out plans for power and has focused mainly on criticising others.
In a bid to professionalise, the party has jettisoned members accused of racism and bullying and distanced itself from ultranationalist movements in other European countries.
Farage said Badenoch's decision to kick Jenrick out of the Conservatives had accelerated talks about him joining Reform, which Jenrick said had begun in September last year.
"I just want to say thank you to Kemi Badenoch," Farage said.
"You've handed me on a plate the man that is by far the most popular figure (in the Conservative Party)."
The Conservatives are the UK's oldest and most successful political party, governing for 32 of the last 46 years.
But their reputation has been badly damaged by a 14-year spell in power that included a divisive Brexit referendum, several chaotic leadership changes and market crises, ending in the party's worst electoral defeat.
Jenrick said he had not always agreed with Farage but described the UK as "on the brink" with welfare, immigration and net zero energy policy among the biggest problems.
