Right back Ashby arrived at St. James' Park from West Ham United during the January 2023 transfer window.
Born in Milton Keynes, Ashby - whose father, Barry, played for Watford, Brentford and Gillingham - came through the youth ranks at West Ham, making seven appearances for the Hammers before joining the Magpies.
Ashby is yet to make his competitive debut for United, but spent the 2023/24 on loan at Championship side Swansea City, where he made 16 appearances and scored once.
He will spend the 2024/25 campaign on loan at Queens Park Rangers in the second tier.