Steffens attended Stanford University, playing on the women's water polo team from 2013 to 2017. Steffens lead the team to NCAA championships in 2014, 2015, and 2017, and to second-place finishes in 2013 and 2016. In 2015, Steffens scored four goals in the semifinal game against USC, which Stanford won, 9-8. In 2017, she scored three goals in Stanford's 8-7 championship victory against UCLA, including the winning goal with 9 seconds left. In 2015 and again in 2017 she was named the MVP of the NCAA tournament. Steffens graduated from Stanford in June, 2017 with a B.S. degree in Science, Technology, and Society.
International career
Her first international appearance was at the 2009 FINA Junior Women’s World Championships in Khanty-Mansiysk, Russia, finishing in the third place. Steffens also made her senior debut at the age of 16, at the 10th Holiday Cup in Newport Beach, California, from December 9 to 13, 2009, where she scored twice. Steffens played on the United States national water polo team which won the 2010 FINA World League Super Final and the 2010 FINA World Cup. She scored the winning goal against Australia in the final match of the FINA World League Super Final. In 2011, Steffens helped the U.S. win the 2011 FINA World League Super Final again. In the Pan American Games, she scored the winning goal in the shootout of the final match, as the U.S. defeated Canada. The U.S. won the FINA World League Super Final for the third straight year in 2012, and Steffens led the team with 11 goals. In the first match of the Summer Olympics, Steffens scored seven goals to tie the Olympic single-game record. She scored 21 total goals in the Olympics, which set a new Olympic record for most goals scored in a women's water polo tournament, as the U.S. won the gold medal. She was named the Best Female Water Polo Player of 2012 by FINA and the 2012 female Water Polo Player of the Year by Swimming World Magazine. Steffens was the top goalscorer at the 2016 Olympics with 17 goals, as the U.S. again won the gold medal.
Steffens was born in San Ramon, California to Peggy Schnugg and Carlos Steffens. She is the youngest of four children. Her father, a native of Puerto Rico, became interested in the sport of water polo after witnessing a match as a child in Puerto Rico. He played for Puerto Rico in three Pan American Games. Her father left the island and joined the water polo team of Berkeley, University of California where he became a three-time All-American, leading the California Golden Bears to the 1977 NCAA championship. In 1979, he was the PAC-10 player of the year in water polo. Maggie Steffens lives in Danville, California. She became interested and was influenced in the sport of water polo mainly by her father and sister, Jessica Steffens, who is also on the United States national team. Steffens became an advisor to women's sports website Just Women's Sports in 2020. She said her goal in working with the company was to increase the awareness of the number of female role models in sports.