Deborah Erskine has been confirmed as the party's new MLA for Fermanagh and South Tyrone to succeed ousted leader Arlene Foster.
The DUP councillor in Fermanagh is a close friend and ally of the former First Minister and had worked in her constituency office.
She previously worked as a DUP press officer and beforehand as a journalist at the Omagh-based Ulster Herald newspaper.
Mrs Erskine was elected in the Erne North area of Fermanagh and Omagh District Council in 2019, with Mrs Foster serving as her election agent in the poll.
Chief electoral officer Virginia McVea confirmed Mrs Erskine's co-option to the Stormont Assembly.
Mrs Foster stood down as DUP leader and Stormont First Minister earlier this year following an internal revolt against her leadership.
It had long been expected that Mrs Erskine, 28, who lives outside Fivemiletown, was set to take on Mrs Foster's MLA seat.
Mrs Foster's departure from the Assembly comes a week after the party she once led celebrated its 50th anniversary.
The former First Minister reportedly declined an invitation to a DUP dinner held at a Belfast hotel to mark the occasion.
At the event new DUP leader Sir Jeffrey Donaldson said the party had "turned a corner" after Mrs Foster's ousting in April sparked unprecedented turmoil within the party.
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