Speaking on the steps of the Senedd in Cardiff on Monday, Farage said: "I've known Helen for many, many years. She is a determined campaigner."
Farage said he was sorry for former leader Thomas's "personal difficulties", but said it had been untenable for him to continue.
Jenner, who served as Thomas's deputy alongside her role as Senedd member for Bangor Conwy Mon, said it was an honour to be elected as Reform Wales leader.
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