She has represented the constituency as a Labour member since 2003.
Mrs Mewies said she looked forward to "new challenges" after working for 50 years, almost 30 of them in politics.
First Minister Carwyn Jones said: "I'd like to thank Sandy for her loyalty and hard work for both her constituency and the Labour party. I wish her the very best for the future."
Delyn AM Sandy Mewies has announced she is to stand down at the next assembly election in 2016.