Sewald attended Bishop Gorman High School in Summerlin, Nevada. In 2008, his senior year, he went 7-0 with a 1.57 ERA, earning All-State honors. Undrafted out of high school in the 2008 MLB draft, he enrolled and played college baseball for the San Diego Toreros at the University of San Diego. As a senior in 2012, he pitched to an 8-4 record with a 3.09 ERA in 15 games. The New York Mets selected Sewald in the tenth round of the 2012 MLB draft. They gave him a $1,000 signing bonus. After signing, Sewald made his professional debut that same season with the Brooklyn Cyclones of the Class A-Short SeasonNew York-Penn League where he was 0-2 with a 1.88 ERA in 28.2 relief innings pitched. He pitched in the 2013 season with the Savannah Sand Gnats of the Class ASouth Atlantic League, compiling a 3-2 record and 1.77 ERA in 35 relief appearances. Sewald began the 2014 season with the St. Lucie Mets of the Class A-AdvancedFlorida State League and was promoted to the Binghamton Mets of the Class AAEastern League during the year. In 43 appearances out of the bullpen between two teams, he was 5-1 with a 1.92 ERA. After the regular season, he played in the Arizona Fall League. In 2015, he spent with Binghamton where he pitched to a 3-0 record and 1.75 ERA in 44 games. During the season, he pitched for the United States national baseball team at the 2015 Pan American Games. Sewald began 2017 with Las Vegas. In 2016, he spent with the Las Vegas 51s of the Class AAAPacific Coast League where he was 5-3 with a 3.29 ERA in a career high 65.2 innings pitched. On April 8, 2017, Mets promoted Sewald to the MLB, as Ty Kelly was designated for assignment. He made his MLB debut the same day against the Miami Marlins at Citi Field and surrendered two runs in a third of an inning. He was optioned back to Las Vegas on April 13 and recalled again on May 1. After his May 1 recall, he spent the remainder of 2017 with the Mets. In 57 relief appearances for New York, he was 0-6 with a 4.55 ERA and a 1.21 WHIP. He was designated for assignment on May 26, 2019. On August 16, the Mets selected Sewald's contract. On September 24, 2019, Sewald earned his first MLB win, as the winning pitcher in a walk-off win against the Miami Marlins.