“I want to thank all the staff and players that I have worked with over the years for their support and hard work on and off the pitch.
“I will always keep a keen eye on how City are doing and wish everyone well for the coming season.”
The Championship season gets under way on 8 September, with Bristol City away to Southampton in their opening game.
Loren Dykes will take on the role of interim head coach as the club search for an immediate replacement.
Gavin Marshall, chair of Bristol City Women, said: “It is hard to put into words how much Lauren has given to Bristol City over the years.
“I thank her for her tireless hard work and commitment to the development of City Women. We are sad to see her go.”
Smith had a short spell as assistant coach for Tottenham Hotspur Women in 2021, between her roles at Wales and Bristol.