Forest manager Mark Warburton had been rumoured to have previously tried to sign Cummings whilst in charge of Rangers, but had been unable to see a deal through. The 21-year-old forward could be key to bolstering the club's attack, having bagged 23 goals in the Scottish Championship last season.
Mahoney, 20 meanwhile has seen his contract expire with Rovers, and has elected to extend his stay in the Championship with Forest.
As Mahoney is under the age of 24, Forest must agree a compensation deal with Blackburn, or face a tribunal. Mahoney would represent a significant capture for Warburton's side, having impressed at Roves during the latter part of last season.
The transfer news will come as a boost to Forest fans, as new owner Evangelos Marinakis demonstrates positive intent in the transfer market.
Former chairman Fawaz Al-Hasawi parted ways following five-year of turmoil, culminating in a final day escape from relegation to League One last season.
Fans will hope that this is just the start of a busy window for the club seeks a squad overhaul ahead of the 2017/2018 campaign.