The 17-year-old centre-back made four senior appearances for Kille last season.
Toon have claimed they beat several clubs to win McArthur's signature.
A United statement read: "United are pleased to confirm the signing of young defender Charlie McArthur from Scottish Premiership side Kilmarnock subject to international clearance. "The 17-year-old centre-back, who captained Scotland at this summer's Under-17 European Championship in Israel, made his senior debut for Killie as a 16-year-old in October 2021 during a Scottish Challenge Cup last-16 win against Queen's Park.
"The promising youngster, who attracted strong interest from a number of Premier League clubs, will link up with United's under-18 side ahead of the 2022/23 campaign."