The Scotland international was named the Reds’ Player of the Season in 2016 and scored 18 goals during her spell on Merseyside.
Weir said: “I have really enjoyed my time at Liverpool Ladies and will take away some great memories. I feel as though I have taken my game to the next level since joining the club and would like to express my gratitude to everyone at the club for helping me to improve as a player.
“However, the chance to join Manchester City Women and play UEFA Women’s Champions League football was too good of an opportunity to turn down.