The 41-year-old was originally expected to finish the season with the Baggies on a 12-month notice period but has instead opted to be available for one day a week as he prepares to begin his career as the FA's director of elite development.
"I have had nine fantastic years here and I just want to say a big thank to everyone connected with the football club - the staff, players and supporters," Ashworth told the club's official website.
"As with any football-related job, there have been some lows but we have managed to bounce back from them all and there have undoubtedly been far more highs."
Chairman Jeremy Peace added: "Dan has worked tirelessly for this football club for the past nine years and leaves with our best wishes.
"He has remained fully committed to his club responsibilities since it was announced last September he would be leaving, for which I would like to thank him."
Richard Garlick was appointed in January and will assume the role left by Ashworth.