Newcomb attended Middleborough High School in Middleborough, Massachusetts. As a senior, he had a 0.46 earned run average with 110 strikeouts in 58 innings pitched. After high school, Newcomb enrolled at the University of Hartford. As a freshman for the Hartford Hawks baseball team in 2012, Newcomb started nine games before suffering a season-ending injury. His first college win was a no-hitter against Yale. He finished the year with a 2–4 win-loss record with a 4.17 ERA and 45 strikeouts in innings. As a sophomore in 2013, he started 12 of 13 games, going 5–4 with a 3.75 ERA and 92 strikeouts over 72 innings. In 2012 and 2013, he played collegiate summer baseball with the Wareham Gatemen of the Cape Cod Baseball League. Newcomb started his junior season without allowing a run through his first innings. He finished the year 8–2 with a 1.25 ERA and 106 strikeouts in innings. He was named the 2014 America East Pitcher of the Year, becoming the first Hawk to win a major conference award.
On November 12, 2015, the Angels traded Newcomb, Erick Aybar, Chris Ellis and cash considerations to the Atlanta Braves for Andrelton Simmons and José Briceño. The Braves invited Newcomb to spring training and he spent the 2016 season with the Double A Mississippi Braves. While pitching in the Southern League, he posted 4.6 walks per nine innings with a 3.86 ERA. Newcomb was invited to spring training for the second time at the start of the 2017 season. He started the 2017 season with the Gwinnett Braves of the Class AAAInternational League. Newcomb pitched to a 2.97 ERA in innings, alongside 74 strikeouts and 33 walks prior to his first promotion to the major leagues. He made his major league debut on June 10, 2017, for the Braves at SunTrust Park against the New York Mets. He pitched innings, allowing four hits and one unearned run with seven strikeouts. For the season, he was 4–9 with a 4.32 ERA. On July 29, 2018, Newcomb took a no-hitter through innings until Chris Taylor singled. The Braves defeated the Los Angeles Dodgers 4–1. After this outing, social media posts Newcomb made to Twitter at the age of eighteen came to light. The comments included homophobic slurs and racial epithets. During the 2018 season, Newcomb tallied an ERA of 3.91, but struggled over his final fourteen starts, recording a 5.50 ERA in that span. Newcomb started the Braves' first game at SunTrust Park in 2019. He faced the Chicago Cubs, pitching four innings of an 8–0 win. Newcomb completed the seventh inning in his next game against the Miami Marlins, but struggled against the New York Mets, leading to his demotion to the Gwinnett Stripers on April 14. Newcomb returned to the major league club on May 4, and made several relief appearances. He returned to the starting rotation to face the Philadelphia Philles on June 15, 2019. Newcomb was struck in the head by a batted ball from J. T. Realmuto, and left the game. He was subsequently placed on the seven-day injured list. Newcomb was reactivated on June 25, and faced the Chicago Cubs as a reliever that night.
