Following a stellar 2019 in which she helped lead the U.S. women's national team to its fourth World Cup, Mewis continued her standout play at club level, first for North Carolina Courage and later for City.
"It's obviously a huge honor to be a part of the list of players that have won this award in the past," Mewis said in a statement.
"It should go without saying that it's the team that does the work on the field and I just have so much respect for the other nominees this year and all my teammates. I've just loved playing with them on the U.S. team, and with my teammates for the Courage and Man City. It's still such an honor to even get called into a U.S. camp, to get playing time and to be able to score goals for my country.
"All that continues to be surreal to me. I owe so much to so many people who have helped me on my journey and any individual award is a credit to them."
After playing five games for the Courage at the National Women's Soccer League Challenge Cup, Mewis moved to Man City, where she has played 17 matches in all competitions, scoring six goals.
In 2020, Mewis played in eight of the nine games for the USWNT, starting six, and scored four goals, all coming at the 2020 CONCACAF Women's Olympic Qualifying tournament.
"I am very happy for Sam," USWNT manager Vlatko Andonovski said. "Due to her work ethic and mentality to be the best player she can be, her every day dedication to being a professional and her sincere caring for her teammates and the team as a whole, she's a true example of what the USWNT stands for.
"She has proven herself and excelled in three different environments this year and I know she has even more great times ahead in her career."
Mewis ended 2020 on a special note, getting to play with her older sister Kristie during the USWNT's 2-0 victory over Netherlands on Nov. 27.
Kristie scored the clinching goal in a match that marked the first time the two had played together for the USWNT in more than six years.
"It's been a strange year with a lot of hardship for a lot of people, but I'm really grateful for the humanity we've seen this year as well," Mewis added.
"I'm just really humbled that I got the opportunity to play for the USA and for two fantastic clubs this year. My family and husband have been so supportive and to even get the opportunity to play soccer this year with everything so uncertain has been a gift.
"I'm grateful for the support of Vlatko and his staff, to the Courage and Paul Riley, who has had a big impact on me, and to all the coaches, staff and players at Man City who welcomed Rose [Lavelle] and I with open arms."
The U.S. Soccer Female Player of the Year has been awarded since 1985, when midfielder Sharon Remer earned the inaugural award. Abby Wambach won it six times, the most of any player.