Simpson joins the Robins after a season in Welsh football with The New Saints.
Before that, he spent two years at Kidderminster Harriers. The 2021/22 season was a particularly memorable one for Simpson, who kept 22 clean sheets during the regular campaign.
His former club only conceded 35 goals in 42 league games during that season as they reached the National League North play-offs.
The 2021/22 season also saw Kidderminster go on a memorable FA Cup journey, where they went without minutes of knocking out Premier League side West Ham United.
The 28-year-old boasts a wealth of Football League and non-league experience, having started his footballing journey with Bury at the age of eight.
The talented shot-stopper also spent time with Leicester City and Watford, who were in the Championship and Premier League respectively at the time.
Ashton United boss Steve Cunningham said: "I believe we have signed one of the best keepers in the north-west.
"This will prove to be one of my best signings as a manager."
Photo credit: Kidderminster Harriers