Millar, who has spent the past two seasons with South Melbourne in the NPL Victoria, has been on trial at the Mariners.
FourFourTwo understands the A-League outfit has offered the 21-year-old a contract for the new campaign and Millar has agreed to it.
The utility, who can play in both midfield and defence, debuted for Dandenong Thunder as a 16-year-old in 2013.
He joined Melbourne City’s National Youth League team two years later, making his senior professional debut in the FFA Cup against Perth Glory.
Millar made four first-team appearances for City before departing for South Melbourne in 2016.
This season he has bagged two goals in 14 matches for the NPL outfit. Millar has featured predominantly as a right winger for Hellas.
New Central Coast boss Mike Mulvey has made eight signings since his appointment in April.
Mulvey has already added Joe Gauci, Kalifa Cisse, Michael McGlinchey, Matt Simon, Corey Gameiro, Mario Shabow, Jack Clisby and Jonathan Aspropotamitis to his roster.