The 26-year-old makes the switch from East Antrim neighbours Ballymena United following the conclusion of his contract at The Showgrounds.
Millar made over 200 appearances for his hometown club and was named Man of the Match in their Irish Cup final defeat to Crusaders last month.
But the midfielder is now looking forward to his new challenge at Larne.
“I’m delighted to be here and now I just can’t wait to get in and meet the boys,” Millar told the club website.
“I’m at a wonderful club and I can’t wait to get out and play in front of the fans and hopefully make them happy.
“The professionalism in and around the club is really noticeable and the change from four or five years ago to now is massive.
“It’s not just on the pitch, but off the pitch as well and hopefully I can come and help to push things on even further.
“Larne are the club who is furthest down the line with the professional approach and they are learning all the time and I think that stands them in good stead.
“The chance to play professional football is massive for me and I can’t wait to get started and give everything for the club.”
Larne boss Tiernan Lynch feels Millar will bring "another dimension" to his side next season.
“We are absolutely delighted to bring Leroy to the club,” he said.
“He has been a huge driving force over the last number of years and everyone saw at the Irish Cup final what he is capable of.
“He helps bring another dimension to the team and the squad and we are delighted to know he will be a Larne player next season.”
Millar's announcement comes hours after Larne had announced they had accepted an "undisclosed offer" from Ballymena for striker David McDaid.
Lynch told the club website they did not want to lose the player, but understood his desire to play regular football.
“Davy has expressed his desire to leave after the conclusion of the last season," he said.
“Coming into his 30’s he wants regular football and I can understand that and respect it.
“Of course we don’t want to lose him and he has played an integral part on the journey we’ve been on for these last five years.
“However we respect his request and we have accepted an offer from Ballymena, who will now be able to discuss personal terms with Davy.”