Los Angeles Angels

Newcomb was considered a top prospect for the 2014 Major League Baseball draft.^[9] He was drafted in the first round, 15th overall, by the Los Angeles Angels of Anaheim.^[10] He signed with the Angels, receiving a $2,518,000 signing bonus.^[11] Newcomb was assigned to the Single–A Burlington Bees and later, in 2015, promoted to the Double–A Arkansas Travelers,^[12][13] where he ended his first full season in professional baseball; he had a combined 2.38 ERA across 3 levels. Finishing 2nd in minor league baseball with 168 strikeouts.^[14]

Atlanta Braves

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.^[15] The Braves invited Newcomb to spring training and he spent the 2016 season with the Double–A Mississippi Braves.^[16][17] While pitching in the Southern League, he posted 4.6 walks per nine innings with a 3.86 ERA.^[18]

Newcomb was invited to spring training for the second time at the start of the 2017 season.^[19] He started the season with the Gwinnett Braves of the Triple–A International League.^[20] Newcomb pitched to a 2.97 ERA in 54+2⁄3 innings, alongside 74 strikeouts and 33 walks prior to his first promotion to the major leagues.^[21][22] He made his major league debut on June 10, 2017, for the Braves at SunTrust Park against the New York Mets. He pitched 6+1⁄3 innings, allowing four hits and one unearned run with seven strikeouts.^[23] For the season, he was 4–9 with a 4.32 ERA.^[24] On July 29, 2018, Newcomb took a no-hitter through 8+2⁄3 innings until Chris Taylor singled. The Braves defeated the Los Angeles Dodgers 4–1.^[25] 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.^[26][27] 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.^[28]

Newcomb started the Braves' first game at SunTrust Park in 2019. He faced the Chicago Cubs, pitching four innings of an 8–0 win.^[29] Newcomb completed the seventh inning in his next game against the Miami Marlins,^[30] but struggled against the New York Mets,^[31] leading to his demotion to the Gwinnett Stripers on April 14.^[32][33] Newcomb returned to the major league club on May 4,^[34] and made several relief appearances.^[35][36] 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.^[37][38] He was subsequently placed on the seven-day injured list.^[39][40] Newcomb was reactivated on June 25,^[41] and faced the Chicago Cubs as a reliever that night.^[42][43]

Newcomb endured an abysmal 2020 season, registering an 11.20 ERA in 13.2 innings pitched across 4 games, striking out 10.^[44]

In 2021 with the Braves he was 2–0 with one save and a 4.73 ERA, as in 32 relief appearances he pitched 32.1 innings, walked 27 batters, and struck out 43 batters.^[45] The Braves finished with an 88–73 record, clinching the NL East, and eventually won the 2021 World Series, giving the Braves their first title since 1995.^[46]

The Braves designated Newcomb for assignment on April 19, 2022.^[47]

Chicago Cubs

On April 20, 2022, Newcomb was traded to the Chicago Cubs in exchange for Jesse Chavez and cash considerations.^[48] After spending time on the injured list with a left ankle sprain, he was activated on June 13. Newcomb allowed five runs in an inning of work in an 18-4 loss against the New York Yankees and was designated for assignment after the game.^[49] On June 20, Newcomb was outrighted to the Triple-A Iowa Cubs.

On August 4, Newcomb was selected back to the active roster to start the second game of a doubleheader against the St. Louis Cardinals.^[50] On September 17, Newcomb was once again designated for assignment by the Cubs. He cleared waivers and was sent outright to Triple–A Iowa on September 19.^[51] He elected free agency on October 6.

San Francisco Giants

On February 6, 2023, Newcomb signed a minor league contract with the San Francisco Giants organization.^[52] In 20 appearances split between the Single–A San Jose Giants and Triple–A Sacramento River Cats, he registered a cumulative 3.15 ERA with 45 strikeouts in 34+1⁄3 innings pitched.^[53]

Oakland Athletics

On August 22, 2023, Newcomb was traded to the Oakland Athletics in exchange for Trenton Brooks.^[54] The next day, Oakland selected Newcomb's contract, adding him to the major league roster.^[55] After posting a 3.00 ERA across 7 appearances for Oakland, Newcomb was placed on the injured list with a sprained left knee on September 17.^[56] On September 20, it was announced that Newcomb had undergone left knee lateral meniscus surgery, ending his season;^[57] he was transferred to the 60–day injured list the same day.^[58]

On November 2, 2023, Newcomb signed a one–year, $1 million contract extension with the Athletics.^[59] On December 20, Newcomb underwent an arthroscopic surgery on his right knee, with the procedure projected to have him ready for game action by spring training.^[60] He was placed on the 60–day injured list to begin the 2024 season after getting behind schedule in his recovery process.^[61]