Keller replaces former Oxford United favourite Chris Allen, who resigned as Millers boss last month after relegation from Southern Premier Division South.
Keller joined the Alfredians` management team in May 2020 as head coach.
The 36-year-old started his playing days with Wantage and spent three years with the club.
He also had spells with Banbury United, Didcot Town, Shrivenham, Carterton, Pontadawe and Southam United.
Michael Hopkins will assist Keller, who finished the 2022/23 season as head of recruitment at North Leigh.
Most clubs are looking for volunteers. Find out more on the button below:
www.PitchingInVolunteers.co.ukAll the news and results in one place.
REGISTER