Brown attended Ward Melville High School in East Setauket, New York. At 17 years old, after getting off to a strong start in his junior baseball season,[1] he suffered a ruptured appendix, was hospitalized for a week and lost 30 pounds (14 kg). After his hospitalization and weight loss, at least five college baseball programs withdrew their scholarship offers to Brown. Nonetheless, he added back 15 pounds (6.8 kg) of muscle and finished his senior year with a 1.18 earned run average.[2] His fastball topped out at 92 miles per hour (148 km/h) as a senior and he committed to play college baseball at Siena.[1]
On November 15, 2022, the Cubs added Brown to their 40-man roster to protect him from the Rule 5 draft.[10] He was optioned to the Double-A Tennessee Smokies to begin the 2023 season, then joined the Iowa Cubs in May.[11] Brown missed a portion of the 2023 season due to an oblique injury.[12] In 26 games split between Tennessee and Iowa, he accumulated an 8–8 record and 4.27 ERA with 100 strikeouts across 72+2⁄3 innings pitched. Brown was optioned to Triple-A Iowa to begin the 2024 season.[13]
Following an injury to Justin Steele, Brown was called up to the major leagues on March 30, 2024.[14] In his debut, Brown allowed five hits and two walks which yielded six runs in 1+2⁄3 innings pitched along with one strikeout against the Texas Rangers.[15][16] On April 9, Brown made his first start, giving up three hits and one walk with five strikeouts in 4+2⁄3 innings in a win against the San Diego Padres.[17][18] In his sixth career start, Brown tossed seven no–hit innings with a career–high ten strikeouts against the Milwaukee Brewers.[19] On June 9, Brown was placed on the injured list with a left neck strain, an injury that was later revealed to be a stress fracture in his neck.[20]