Ashworth agreed to see out his role as Brighton's technical director until the end of the transfer window out of respect to the Seagulls.
Brighton say he will now serve out gardening leave before joining another Premier League club.
Ashworth held talks with Newcastle at the end of last year after they were taken over by a Saudi-backed consortium.
Now he's agreed to join the Magpies as their director of football, where he will work closely with manager Eddie Howe on all football matters.
Ashworth joined Brighton as technical director in 2018, having previously held the same role with the Football Association.
He was widely praised the job he did in charge of England's elite men's and women's teams and was responsible for the appointment of Gareth Southgate as manager.
Brighton chairman Tony Bloom said: "We are extremely disappointed that Dan will no longer be our technical director. He leaves a significant legacy in place and for that we are greatly appreciative.
"Dan's done an outstanding job and helped build on the progress the club had already made across all of our technical areas. I wish Dan and his family well for the future."
Newcastle are currently battling to avoid relegation to the Championship.
They are sitting 19th in the Premier League table, a point adrift of safety having played a game less than 17th-placed Norwich.