As an amateur she won a gold medal in the women's middleweight division at the 2012 and 2016 Olympics, making her the first American boxer—female or male—to win consecutive Olympic medals. Shields was the youngest boxer at the February 2012 U.S. Olympic trials, winning the event in the 165 lb (75 kg) middleweight division. In May, she qualified for the 2012 Olympics, the first year in which women's boxing was an Olympic event, and went to become the first American woman to win an Olympic gold medal in boxing.
Shields was born and raised in Flint, Michigan, where she was a high school junior in May 2012. She was introduced to boxing by her father, Bo Shields, who had boxed in underground leagues. Bo was in prison from the time Shields was two years old, and released when she was nine. After his release he talked to her about boxer Laila Ali, piquing her interest in the sport, although Bo nevertheless believed that boxing was a men's sport and refused to allow Shields to pursue it until she was eleven. At that time she began boxing at Berston Field House in Flint, where she met her coach and trainer, Jason Crutchfield. Shields credits her grandmother with encouraging her to not accept restrictions based on her gender.
After winning two Junior Olympic championships Shields competed in her first open-division tournament, the National Police Athletic League Championships 2011; she won the middleweight title and was named top overall fighter as well as qualifying for the U.S. Olympic trials. At the trials in she defeated the reigning national champion, Franchon Crews, the 2010 world champion, Andrecia Wasson, and Pittsburgh's Tika Hemingway to win the middleweight class. In April 2011, she won her weight class at the Women's Elite Continental Championships in Cornwall, Ontario against three-time defending world champion Mary Spencer of Canada; she held an undefeated record of 25 wins and 0 losses at that point.
Following Shields' victory at the U.S. Olympic trials, it was initially reported that she would need only a top-8 finish at the 2012 AIBA Women's World Boxing Championships in Qinhuangdao, China, in order to qualify for the 2012 Olympics. On May 10, the day after the contest began but before Shields' first bout, a change to the rules was announced that meant Shields would need to place in the top two from the (North, Central, and South) American Boxing Confederation region of AIBA (AMBC). Shields won her first round, but suffered an upset loss in the second round on May 13 to Savannah Marshall of England, bringing Shields' record to 26-1. Her chances for qualification thus depended on Marshall's subsequent performance; after Marshall advanced to the middleweight finals on May 18, it was announced that Shields had earned an Olympic berth. She won a gold medal in the end, after beating Russian boxer Nadezda Torlopova 19–12.
In 2014, Shields won the World Championships and the following year, she became the first American to win titles in women's boxing at the Olympics and Pan American games. Shields won gold at the 2016 AMBC Olympic Qualifying tournament in Argentina. Later that year, she won the gold at the 2016 Rio Summer Olympics by defeating Nouchka Fontijn of the Netherlands, the only American gold medalist in the team, and awarded the inaugural women's division of the Val Barker Trophy at the competition.
Her amateur boxing record was 78 wins (19 by knockout) and 1 loss.
In November 2016, Shields officially went pro. She won her first match, against Franchon Crews, by unanimous decision.
On March 10, 2017 she faced Szilvia Szabados for the North American Boxing Federation middleweight title, and won. This was the main event on ShoBox, with a regional title fight between Antonio Nieves and Nikolai Potapov serving as the co-main event. It was the first time a women's boxing bout was the main event on a United States premium network card.
On June 16, 2017, Shields headlined the "Detroit Brawl," facing Sydney LeBlanc in her first scheduled eight-round bout. LeBlanc signed on with three days notice, after Mery Rancier dropped out due to visa issues. Shields won the bout by decision after all eight rounds.
On August 4, 2017, Shields defeated defending champ Nikki Adler in Detroit for the WBC super-middleweight belt and the vacant IBF super-middleweight belt. The fight was on Showtime.
While growing up in Flint, Michigan, Shields was sexually abused by her mother's boyfriends. Shields was baptized at age 13 (two years after she began boxing) and began attending a local church. She found strength in her Christian faith and eventually left home. Now Shields prays before every fight, talks about God's plan for her life, and says, "All glory to God."
Shields attempted to adopt her cousin's daughter in 2014.
Shields is an ambassador for Up2Us Sports, a national non-profit organization dedicated to supporting underserved youth by providing them with coaches trained in positive youth development.
Shields is the subject of the 2015 documentary "T-Rex: Her Fight for Gold." In 2016 Universal Pictures, a division of Comcast, which holds Olympic broadcast rights in the United States, acquired the rights to produce a film about her life story. Barry Jenkins is the screenwriter.
Shields will be acting in the Susan Seidelman-directed film Punch Me.
In 2017, Shields won the Nickelodeon Kids' Choice Sports Award for "Biggest Powerhouse."