The length of the deal is undisclosed but sources have told ESPN that Mewis will be at the club for the "foreseeable future."
"I know the Super League has just been growing so much and has such a great competition and has a lot of world class players playing here so I thought it sounded like such a great opportunity to develop my game in a new way and get outside my comfort zone a little bit," Mewis told ESPN in an exclusive interview.
"I think that all the players here are so comfortable on the ball and so technical and I'm sure have just such a great tactical understanding of the game.
"I know it is going to be really competitive and I'm going to have to earn some playing time because the midfield here is really talented and really good but I'm excited for the challenge and I hope that living here and being part of this culture is going to add some depth and experience to my game which I'm excited for and is one of the reasons I came."
Mewis said that while the decision to switch leagues is a personal one, she is optimistic that it is a reflection of the growing global nature of the women's game.
"I think it is such an individual decision to switch teams or change leagues and the opportunity is so great that I personally was like this is a chance for me to add some depth to my game and continue to evolve as a player," she added.
"Just to be here in England where the culture of football is so incredible. I hope that as women's soccer and the whole world continues to involve and grow that players get opportunities like this because I do think that playing anywhere that you can and just having these new experiences to add to your tool belt will make us better players."
Mewis has won the National Women's Soccer League three times with the Western New York Flash in 2016 and the Courage in 2018 and 2019.
"I have nothing but good things to say about the NWSL," Mewis said of the tournament. "I think they're in such good hands and I think that the Challenge Cup went off so well and I was really happy to be a part of it to get some good training and games in during a time where people couldn't.
"I know I have to wear a mask all the time and we're going to have to get tested and just that understanding of not only am I keeping myself safe and healthy but I'm responsible for keeping others safe and healthy. I think that was a good lesson for me to learn and bring here."
"I'm happy for Sam and I know she has the high level of professionalism and maturity necessary to move to a new environment and excel in the next stage of her career. She'll get some valuable games in England, and we'll of course be watching," USWNT coach Vlatko Andonovski said.
Mewis has made 67 international appearances, registering 18 goals and six assists.
City finished second in the 2019-20 Women's Super League (WSL), which was cancelled in May due to the coronavirus pandemic.
Despite City leading the league in points when it was suspended in March, Chelsea had a game in hand and were declared eventual champions on a points-per-game basis.
The 2020-21 season marks the first in charge for manager Gareth Taylor after Nick Cushing, who led the side for six years, left for New York City FC in January.
City will start their campaign on Sept. 6 away to newly promoted Aston Villa.
Alongside a WSL title, City will be aiming for Champions League glory in their fifth consecutive season in the European tournament.
The club have reached the semifinals twice, losing to Lyon both times.