Harney will cross to the Murray Football League outfit in 2022 alongside another Elmore player in Bailey Boyack.
They follow one-time Bloods Farran Priest and Phil Hubbard to Echuca South Recreation Reserve, after Priest took on the senior coaching role ahead of the 2022 season.
Hubbard quickly signed at United, having co-captained the Eagles alongside Priest in 2019.
Priest said he was excited to get both Harney and Boyack across to the club next season.
“James and Bailey are very good players,” Priest said.
“James has been a leader at Elmore for some time and is a very good forward — he's a general star, so I'm very keen to see him play at United.
“And Bailey, he's a bit younger and is willing to learn. He's strong, and loves to have a crack.
“He's going to be a good one to watch and a big asset for us.”
An inside and outside midfielder, Boyack will roll through the engine room, with Priest crediting his tank.
He has made 35 senior appearances for the Bloods since 2017, having played at Elmore Recreation Reserve since his under 17 days.
Harney has also been around the club since 2009 when he first began playing with its under 17s side.
The forward lined up for 13 games this season and kicked 32 goals.
His leadership skills will be pivotal at United, which is going through a rebuilding phase with a host of young talent.
“He captained a young side at Elmore and definitely knows what he is doing.”
Priest has re-signed a large part of that young group, and expects to add more talent to the group over the next couple weeks.
United and Elmore's off-season link has seemingly gone the other way too, with Eagles midfielder Kyle Armstrong signing with the Bloods in recent weeks.