Jenner had been interim leader and favourite to replace Thomas, with sources telling Wales that “very senior” Reform figures had backed her.
She is a married mother-of-two from Anglesey and had spent 15 years working as a teacher.
A fluent Welsh speaker, Jenner had been a member of all Farage’s parties since UKIP, and stood for the Brexit Party in the 2019 Senedd election.
Thomas stepped down on 15 September after he was arrested on suspicion of assault and controlling or coercive behaviour. He was bailed pending further enquiries, and denies doing anything wrong.
He was not on the steps of the Senedd for Jenner’s unveiling as leader, but he remains a Reform MS after chief whip Llŷr Powell said he had not considered removing the whip from him.
After he stepped down, Thomas had been among the Reform Senedd members who met in Cardiff Bay last week to discuss their next steps, with a party official from London attending to oversee proceedings.
Nigel Farage told reporters that it was “pretty much untenable for him to continue as leader”.
He described Jenner as “someone I trust”.
Farage added the party will carry on “as if nothing had happened, waiting for Dan, who is going to stay a member of this place, to come back more actively with us”.
Speaking to Wales, Jenner said there were a “few things” Reform needed to change, including giving members more of a say on policy.
She denied the party had a “unity issue” after James Evans claimed last week he would have had “majority” support to stand as leader had he not ruled himself out.
“I think that it was a really difficult time for us,” Jenner said.
“They say a week’s a long time in politics – to be honest, 24 hours was a long time in politics. But I think we are really united in terms of what we’re trying to achieve here.”