The Vanarama National League South side have acted quickly after Sam Beasant left the club.
Warner has previously played for the likes of Woking, Charlton Athletic, Aldershot Town and AFC Wimbledon.
The keeper has Football League experience in League's One and Two.
He also played a big part in Sutton United promotion to the Vanarama National League and also the Emirates FA Cup run they went on during the 2016/17 season.