The 44-year-old has spent the past five years in charge of Tonga, leading them to a World Cup semi-final in 2017.
He had been working as assistant to ex-Saints boss Nathan Brown at NRL side Newcastle Knights and became interim head coach when Brown left in August.
Woolf has signed a two-year deal with Super League leaders Saints, with a further 12-month option.
"I'm confident we can achieve great things together," Woolf said. "I'm excited and privileged to be joining St Helens as head coach. It's a great opportunity for me."