After the WPS later suspended operations in early 2012 due to legal and financial difficulties, Morgan joined her national teammates Hope Solo, Sydney Leroux, Megan Rapinoe and Stephanie Cox on the Seattle Sounders Women for the 2012 season. Of her signing, Morgan said, "I am excited to play in a city that is so passionate about soccer. The Sounders have been one of, if not the best fan support in MLS. I can only imagine how Seattle fans would respond to having a full professional women's team in the future."
Morgan did not become a starter for the U.S. until the fifth match of the year in January, the final of the CONCACAF Olympic qualifying tournament. She scored twice and provided two assists to Wambach that day, in a 4–0 win over the Canadian hosts and has become a regular starter ever since. Between January and late May 2012, Morgan scored 14 goals in a 12-match stretch including a sequence of three straight two-goal efforts on January 29, February 11 and 29. She earned her first career hat-trick on March 7, 2012, during a 4–0 victory against Sweden in the third-place match of the Algarve Cup. In June 2012, she was nominated for an ESPY Award as the Best Breakthrough Athlete.
Morgan scored the winning goal in the Olympic semi-final match against Canada in extra time, sending the United States to the gold medal match against Japan. Her goal came in the 123rd minute, the latest goal ever scored by a member of the U.S. women's team and a FIFA record. The goal continued her propensity for late heroics in the closing stages of matches. Seventeen of her 28 total goals to date have come after the 60th minute. The match-winning goal was Morgan's team-high 20th in 2012, becoming only the sixth and youngest U.S. player to do so in a single year. In the final, a 2–1 win against Japan on August 9, Morgan assisted on a Carli Lloyd header. She ended the tournament with three goals, and a team-high four assists (tied with Megan Rapinoe) and ten points (tied with Rapinoe and Wambach). To celebrate her achievements, she was honored at her former high school and the No. 13 jersey was retired.
In 2012, Morgan led the U.S. in goals (28), multi-goal matches (9), assists (21) and points (77). Her calendar year goals, assists and points totals are the third-best, tied second-best (one tally shy of record), and second-best, respectively, in USWNT history. Morgan joined an exclusive club as she and Hamm are the only USWNT players to record at least 20 goals and 20 assists in the same calendar year and became only the third and youngest player to reach 20 assists in a calendar year. Morgan and Wambach combined for 55 goals in 2012 – matching a 21-year-old record set in 1991 by Michelle Akers (39 goals) and Carin Jennings (16 goals) as the most goals scored by any duo in USWNT history. She had either scored or assisted on 41 percent of the USA's 120 goals this year. And, by herself, she comfortably out-scored and out-assisted her opponents, who combined for 21 goals and 12 assists in 32 matches against the U.S.
In 2012, Morgan signed with Simon & Schuster to pen The Kicks, a four-book series for middle schoolers. The series is focused on four young girls and features themes of friendship, leadership, and soccer. In a statement released by the publisher, Morgan said she wanted her books to "inspire young girls" and "celebrate" her love of soccer. The first novel, Saving The Team was released on May 14, 2013, followed by the second novel, Sabotage Season, on September 3, 2013. Saving the Team debuted at number seven on The New York Times Best Seller list for Children's Middle Grade. In 2015, a live-action kids comedy series based on the books, called The Kicks, began airing on Amazon Prime.
Morgan has been featured in a number of magazines. In the 2012 Sports Illustrated Swimsuit Issue, she appeared in a section composed of athletes in body paint. She was featured in the 2013 music issue of ESPN The Magazine replicating Katy Perry's One of the Boys album cover. In May 2015, Morgan was featured on the cover of ESPN Magazine with teammates Abby Wambach and Sydney Leroux. The same year, she appeared on multiple covers of Sports Illustrated before and after winning the 2015 FIFA Women's World Cup. She appeared for a second time in the Sports Illustrated Swimsuit Issue in 2015. Morgan posed for one of the three 2019 Sports Illustrated Swimsuit Issue covers, the other cover models being Tyra Banks and Camille Kostek. She has appeared on the covers of Health and Self magazines. She has been featured in Shape, Vogue, Elle, Time, and Fortune.