Mahoney's Blackburn Rovers boss, Tony Mowbray, had expected the winger to join Mark Warburton's men, but the lure of the Premier League looks to have proved too strong.
Blackburn Rovers' Connor Mahoney
But could Forest's disappointment lead to opportunity, with Mahoney's arrival set to swell Bournemouth's army of players in his position to seven - the youngster joining Marc Pugh, Max Gradel, Josh King, Junior Stanislas, Ryan Fraser and Jordon Ibe at Dean Court.
At least one of those could end up on the market this summer, with Pugh and Gradel the two prime candidates if King isn't lured elsewhere - although, of the two, Pugh is perhaps the more realistic Forest target, with Gradel costing around £8 million two summers back.
The Englishman - who, like Mahoney, can play on either flank - started less than half of Bournemouth's Premier League matches last season and is out of contract at the end of the coming campaign, so selling him now is in the Cherries' best interests if they don't plan on keeping him beyond then.
His contract situation could also bring the price down for Forest, who would have had to pay for Mahoney anyway - despite his contract expiring at the end of June - because of his age.
Bournemouth's Marc Pugh
And given Pugh's experience in the top two tiers, he'd be an arguably better - at least in the short-term - albeit significantly older - a decade - alternative to the 20-year-old.
Would Pugh be a good buy for Nottingham Forest?