Newt opposing the Dark Lord is never mentioned in the series.
Though it’s possible that Newt could have been in the opposing forces against the Dark Lord, he’s never mentioned as taking part in the fight against him in any way. However, there are likely some opponents of the Dark Lord that aren’t mentioned specifically, so it’s not certain that he didn’t.
Newt has only been previously mentioned as a textbook author.
Newt’s name has only ever been mentioned once in the seven Harry Potter books, and that was as the author of Fantastic Beasts and Where to Find Them, a textbook on the Hogwarts book list.
All students should have a copy of each of the following:
The Standard Book of Spells (Grade 1) by Miranda Goshawk
A History of Magic by Bathilda Bagshot
Magical Theory by Adalbert Waffling
A Beginner’s Guide to Transfiguration by Emeric Switch
One Thousand Magical Herbs and Fungi by Phyllida Spore
Magical Drafts and Potions by Arsenius Jigger
Fantastic Beasts and Where to Find Them by Newt Scamander
The Dark Forces: A Guide to Self-Protection by Quentin Trimble”
- Harry Potter and the Philosopher's Stone, Chapter 5 (Diagon Alley)
He’s also the in-universe author of the companion book Fantastic Beasts and Where to Find Them and a few more details about him are given in the foreword as that’s supposed to be his book, but he’s not mentioned as helping fight the Dark Lord in that either.