Cardiff were in negotiations to sign the midfielder after agreeing a fee with Toulouse but the 41-year-old says the Frenchman will now join Spurs.
The Bluebirds were thought to have been favorites to win the signature of the France international after agreeing a fee for the 25-year-old in July and entering negotiations for personal terms.
However, Toulouse president Olivier Sadran revealed on Wednesday that the player will join Spurs and Mackay confirmed the news when asked about the deal to sign the midfielder.
"We had a bid accepted but it looks as if he has decided, as far as I am aware, to go to Tottenham. It looks like it will go that way," Mackay told reporters.
Capoue looks set to join the likes of Nacer Chadli and Robeto Soldado to have arrived at White Hart Lane this summer.