Sonnett, along with her sister, began playing for North Atlanta Soccer Association (NASA) in 2003. With NASA, the Sonnetts earned back-to-back Georgia State Cup Championship titles in 2007 and 2008 and also the Region III League Championship title in 2008. In the 2011 US Youth Soccer National Championship Series, NASA 12 Elite II placed third in U-17 standings, and Sonnett was named to the U-17 Best XI as a midfielder.

In 2011 during Sonnett’s junior season, Fellowship Christian’s varsity girls soccer team reached the Final Four of the Class A state tournament, losing 0–2 to the eventual state champions, First Presbyterian Day School, in the semi-final. The following year, Fellowship Christian, with Sonnett as captain, reached the 2012 Georgia state championship game and again faced First Presbyterian Day School ultimately losing 0–1. Sonnett ended her career at Fellowship Christian as the school’s leading scorer with 113 goals and 41 assists.

In April 2011, Sonnett participated in the Southeast’s Region III Olympic Development Program in Costa Rica. Sonnett received her first call-up to the United States U18 training camp as a midfielder in June 2011.

Sonnett attended Fellowship Christian School in Roswell, Georgia, graduating in 2012.

In her first season in 2012, UVA Cavaliers won the regular season Atlantic Coast Conference (ACC) title and made it to the round of 16 in the 2012 NCAA Women’s Soccer Tournament. Sonnett herself was named to the ACC All-Freshman and All-Tournament Team. Though she was recruited as a center midfielder, Sonnett transitioned to the center back position early in her freshman year due to various injuries leaving the UVA back line bare and played in that position the remainder of her college career.

In 2013, Sonnett, playing in the midfield position, helped NASA 12 Elite II win the U-19 national title at the 2013 US Youth Soccer National Championship Series in Overland Park, Kansas. At the conclusion of the tournament, she was named to the U-19 Best XI and also earned the Golden Ball award as the best player at the tournament.

Sonnett received call-ups for the United States women’s national under-23 soccer team training camps in April and December 2013. She later represented the United States as a center back at the U23 level during the 2014 Six Nations Tournament in March 2014. Sonnett appeared in all three games, helping the team ultimately win the tournament title in the final against Norway.

In 2015, Sonnett was named team captain at the beginning of her senior year. Mid-season in October 2015, Sonnett earned her first call-up and started her first cap (as center back) for the senior United States women’s national soccer team against Brazil.

In February 2015 Sonnett again appeared in all three games in the 2015 U-23 Six Nations International Tournament in La Manga, Spain helping the team earn their second consecutive tournament title and serving as captain in their match against Norway. Although Sonnett was called up to the USWNT U-23 training camp in Lakewood Ranch, Florida in April 2015, she was not named to the roster for the 2015 Four Nations International Nordic Tournament held in May 2015.

On October 16, 2015, Sonnett received her first senior call-up to the U.S. women’s national soccer team (USWNT) from Jill Ellis. She received her first cap and start (while still a student at UVA) for the USWNT on October 25, 2015 at center back during a friendly against Brazil as part of the USWNT’s 2015 FIFA Women’s World Cup victory tour. She earned three more caps with the USWNT before the end of 2015 in friendlies against Trinidad and Tobago and China.

On January 15, 2016, Sonnett was selected by Portland Thorns FC as the first overall pick in the 2016 NWSL College Draft. She made her debut with the Thorns on April 17, 2016. Her first professional goal came in the NWSL championship semi-final to bring the Thorns level with the Western New York Flash, though the Flash would go on to score the winning goal and ultimately win the championship.

In January 2016, Sonnett joined the national team for their first training camp of the year at the U.S. Soccer National Training Center in Carson, California and earned her second start in a friendly against the Republic of Ireland that same month. In February 2016, Sonnett was named to the USWNT roster for the 2016 CONCACAF Olympic Qualifying tournament, playing in two of the five games in the tournament. The USWNT’s semi-final win over Trinidad and Tobago qualified the team for the 2016 Rio Olympics. The USWNT then went on to win CONCACAF title for the fourth consecutive year.

Sonnett was named to the roster for the 2016 SheBelieves Cup that took place in March 2016. Sonnett played the full 90 minutes in the tournament’s opening match against England on March 3, 2016. The USWNT would go on to win the 2016 SheBelieves Cup after defeating Germany 2–1 in the final.

In July 2016, Sonnett was named an alternate for the USWNT at the 2016 Rio Olympic Games, along with Ashlyn Harris, Sam Mewis, and Heather O’Reilly. The USWNT drew against Sweden in the quarterfinal but ultimately lost in a penalty shootout, 3–4, resulting in elimination from the tournament.

Sonnett scored again in the NWSL post-season in the 2017 season, helping the Thorns to a 4–1 win over the Orlando Pride in the semi-final. They would go on to win the championship, with Sonnett tallying an assist on Lindsey Horan’s game-winning goal.

On October 24, 2017, it was announced that Sonnett signed for Sydney FC for the 2017–18 W-League season. In her first appearance, she scored a penalty kick. During the semifinal against Newcastle Jets, she made a box to box run to assist Lisa De Vanna’s game-winning goal in extra time.

In 2018 Sonnett appeared in 22 games for Portland, scoring one goal. Portland reached the NWSL Championship Game for the second straight year, but lost to North Carolina 3–0. She was named to the NWSL Team of the Month for March and April. Sonnett was one of four Portland Thorns named to the NWSL Best XI and was a finalist for Defender of the Year.

In January 2018, Sonnett was called up to her third consecutive January training camp with the USWNT. She then returned to the pitch for the USWNT as a substitute for Taylor Smith at left-back in the January friendly against Denmark, earning her 13th cap for the USWNT (and making her first appearance in well over a year).

In February she attended the pre-SheBelieves Cup training camp and was ultimately named to the 23-player roster for the 2018 SheBelieves Cup for the third consecutive year. Sonnett appeared in two of the three USWNT matches in the tournament, and made her first start in over a year and played the full 90 minutes against England in the tournament’s final on March 7, 2018. In April she earned two starts in a pair of friendlies against Mexico but was left off the roster for the friendlies against China in June.

In April during the friendly against Australia, Sonnett started as right-back and registered two assists to Tobin Heath’s goal in the 53rd minute and Mallory Pugh’s in the 67th minute. She remained on the USWNT roster and appeared as a substitute in the World Cup send-off matches against South Africa and Mexico in May 2019.

On May 2, 2019, Sonnett was named to the 23-player roster that represented the United States at the 2019 FIFA Women’s World Cup. Sonnett earned one cap in the second group stage game against Chile, entering the game at the 82nd minute to replace Abby Dahlkemper. The United States went on to defeat the Netherlands 2–0 in the final, winning back-to-back World Cup titles and the fourth overall World Cup title for the United States women’s national team.

On January 8, 2020, Sonnett was traded to Orlando Pride along with the NWSL rights to Caitlin Foord and two draft picks in exchange for Orlando’s No. 1 overall selection in the 2020 NWSL College Draft. In March 2020, the NWSL announced an indefinite delay on the start of the 2020 NWSL season due to the COVID-19 pandemic. The NWSL eventually began with the smaller schedule 2020 NWSL Challenge Cup tournament in June. However, on June 22, Orlando Pride withdrew from the tournament following positive COVID-19 tests among both players and staff.

On August 18, 2020, having been unable to play for Orlando, Sonnett joined Swedish Damallsvenskan club Kopparbergs/Göteborg FC on a short-term contract until November 2020. Orlando retained Sonnett’s NWSL rights. Five days later she made her Damallsvenskan debut, starting in a 3–0 defeat to Rosengård. She made 9 league appearances as Göteborg won its first Damallsvenskan title.

Sonnett attended her fifth consecutive January training camp that was held in Tampa, Florida, the first lead by Andonovski. Following the January camp, she was named to the 20-player roster for the 2020 CONCACAF Women’s Olympic Qualifying tournament held in late January/early February 2020. Sonnett appeared in 4 of the 5 matches during the CONCACAF tournament as right back (switching to left back with substitutions of Kelley O’Hara and Ali Krieger). The USWNT qualified for the 2020 Olympics with their win against Mexico and were named champions at the conclusion of the 2020 CONCACAF women’s tournament. Sonnett was also named to the 23-player roster for the 2020 SheBelieves Cup and played all 90 minutes in the match against Spain at left back, aiding the USWNT to ultimately capture the tournament title on March 11, 2020.