"City is one of the world's best clubs and I've heard so many good things about the women's team and their incredible training facilities," the 27-year-old told the club's website.
Mewis has scored 18 goals in 67 international appearances for the United States and at club level has won three NWSL titles, one with Western New York Flash and two with North Carolina.
Manchester City led the Super League when the season was interrupted due to the corona pandemic this spring. Despite that, the league title went to Chelsea, who had a better scoring average.