Fainga'a scored 11 tries last year, third behind Sevu Reece (15) and Ngani Laumape (13).
Fainga'a's contract was until the end of the 2020 season but he made the extension although he said it wasn't an easy decision.
"All my family is in Sydney, so to lock myself in here until 2022 by myself wasn't easy. But the Brumbies are my family away from my family back in Sydney," he said.
Fainga'a told rugby.com.au that he knew he had made the right choice.
"The group here – they make it easier for me and coaching staff as well with Dan [McKellar], Lord, [Laurie Fisher] and Hewy [Pete Hewat], we've got great staff around and support as well," he said.
"The players and the leadership around me as well…help me around my rugby and focus."
Fainga'a has made a quick impact in the two years he has been with the Brumbies. Originally the Sydney University No.2 hooker, he was now a Test squad regular for Australia.
The 24-year-old still pinches himself with how fast his career has moved.
"Two years ago I was working on a construction site and now I get to do the job that I've dreamed of," he said.
"The year's just gone fast and in the blink of an eye you're at the end of the season and you're ready to rip into a new one, so, no, I have not really [reflected], everything's come around so quick."
Being part of his first Rugby World Cup campaign had topped his achievement.
"The vibe there [in Japan], the experience, especially for myself, I keep pinching myself, I didn't think I was going to be in that squad.
"I was still really new to the international stage but to have guys like Tolu [Latu] and Jordan [Uelese] around me, it helped me go through the World Cup stage. It was awesome," he said.
Latu's move to Stade Francais is likely to see him get more opportunities at Test level this year and he had returned to training early with the Brumbies to help his chances.
But he acknowledged there was plenty of competition for a place in Dave Rennie's squad.
"You've still got world-class hookers in Australia, guys like Jordan [Uelese], Anaru [Rangi], Brandon [Paenga-Amosa] and Alex Mafi up in Queensland and myself here with Connal [McInerney] and young [Lachlan] McInerney and [Billy] Porter coming through the system as well.
"Australia's in very good hands with hookers and competition's healthy," he said.