Flanagan will become part of St George Illawarra's coaching staff as an NRL assistant coach in late December.
The club released a statement on Wednesday to say it had "exercised due diligence throughout the lengthy appointment process of Flanagan in consideration of his 2018 NRL coaching deregistration".
The former premiership-winning Sharks mentor will join the Dragons on a one-year deal and the NRL has informed the club that he can begin his role with the club next month.
