Brentford are long-term suitors while Newcastle, Leeds, Leicester and Tottenham were also interested in the Welshman.
But Johnson has stayed loyal to Forest after dreaming of playing for the club in the top-flight.
Johnson said of his new four-year deal: "Just wanted to say that firstly the support that I've felt from the fans last season was unbelievable.
“It has been an unbelievable journey from the Academy to the first team and I want to thank the manager, all the staff and the whole board for believing in me and giving me the confidence to go out on the pitch and show what I can do.
“I'm really excited for this new season and feeling the energy from the fans out on the City Ground which I'm sure will be something special."
Steve Cooper, who turned down an £18million bid from Brentford for Johnson last season, said: “Brennan was instrumental in our success last year and he's got the potential to make a big impact in our return to the Premier League.
“He's spent his entire footballing life with Nottingham Forest and everyone at the club is very proud at what he's achieved to date.
"However, at just 21-years-old, his best days are ahead of him and he has a very high ceiling at what he could achieve.
"With that in mind, we're all delighted that he has committed himself to Nottingham Forest long-term."