Everton and Boro agreed a £25m deal for the 24-year-old earlier this week, and Hackney is now putting the finishing touches to his move to the Hill Dickinson Stadium.
It's understood Everton will pay around £17m up-front for the former England Under-21 international, with the rest in add-ons.
The move will bring to an end Hackney's long association with his boyhood club. The midfielder has played more than 150 games for Boro and was named the Championship's Player of the Season last term.
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With Middlesbrough missing out on promotion and Hackney entering the final year of his contract at the Riverside, an exit was always extremely likely this summer, though Boro made it clear that they would only allow their star man to move on if their valuation was matched.
Boro's players are due to report back to Rockliffe on Thursday for the start of pre-season, but Hackney obviously isn't expected to be present, with Everton confirmation now imminent.
Everton were one of more than half a dozen Premier League clubs who were interested in Hackney this summer, though the Toffees were always in a strong position with the midfielder having indicated a move to Merseyside was his preference.
Tottenham, Nottingham Forest, Crystal Palace and Hull City were among the other clubs interested, while Michael Carrick - Hackney's former boss at Boro - remains an admirer.
After reporting back for pre-season, Boro's players will spend a couple of days at Rockliffe before heading out to Portugal this weekend for a training stint.
As things stand, Dael Fry won't be among the players reporting back on Thursday, amid remaining uncertainty surrounding the defender's future. Fry is now officially out of contract at the Riverside and talks with Boro are understood to be continuing.
There is interest from elsewhere in the Championship, with West Ham monitoring the defender's situation and Blackburn having also been linked.