Previously, Mackay was a player agent for London-based Stellar Group.
United and Nashville SC officials declined to comment.
Mackay will become the most powerful figure on United’s technical side, tasked with setting the team culture and leading the search for a head coach following Wayne Rooney’s departure in October.
Mackay will essentially replace Dave Kasper, who, with multiple titles over almost 22 years, has supervised most roster moves. Kasper is currently United’s sporting director and president of soccer operations. A salary cap expert, he is expected to become a senior adviser to chief executive Jason Levien, though without decision-making powers.
Lucy Rushton was general manager under Kasper in 2021 and 2022 before being fired last fall. This season, Kasper and technical director Stewart Mairs oversaw personnel decisions.
Unlike Rushton, Mackay will report directly to Levien, not to Kasper, multiple people familiar with the club’s plans said.
United interviewed several other candidates from MLS teams, as well as Fredi Bobic — the former sporting director of Bundesliga clubs Stuttgart, Eintracht Frankfurt and Hertha Berlin — and at least one executive from a Liga MX club in Mexico.
United has not qualified for the postseason since 2019 and hasn’t won a playoff game since 2015. Next season will mark the 20th anniversary since the club’s last MLS Cup title.
This season, United faded down the stretch and finished 10-14-10 for 12th place in the Eastern Conference and 23rd overall in a 29-team league. Last year, its 7-21-6 record was MLS’s worst.
Mackay’s top priority will be replacing Rooney, who, four days after leaving United, was hired by Birmingham City in England’s second division. He brought along two D.C. assistants, Pete Shuttleworth and Carl Robinson.
According to people close to the situation, goalkeepers coach Diego Restrepo left to fill the same position for Bay FC, a 2024 NWSL expansion team, whose GM is Rushton.
As part of his discussions with United management, Mackay has already engaged in talks about possible coaching candidates from MLS circles and overseas, one person said.
In coordination with the new coach, Mackay will also need to identify a third designated player, a slot that opened when United parted ways with Greek forward Taxi Fountas late in the season following a second allegation of using a racial slur.
United’s other designated players, striker Christian Benteke and midfielder Mateusz Klich, are expected to return. Designated players are high-cost talent who are largely exempt from the league’s salary cap.
With Mackay in place, the club is expected to soon announce decisions on players with contract options. United is aiming to re-sign free agent center back Donovan Pines, who is seeking opportunities in Europe. It must also decide whether to retain playmaker Gabriel Pirani, whose loan from Santos in his native Brazil is expiring.