In 2017, Trejo attended California State University, Northridge to play college soccer. In three seasons with the Matadors, Trejo made 58 appearances, scoring 26 goals and tallying 15 assists. Trejo's achievements at CSUN during his time there include 2019 Big West Offensive Player of the Year, 2019 CSUN Big West Scholar-Athlete of the Year, 2019 Second Team All-Region, 2019 First Team All-Conference, 2019 Academic All-Conference, 2018 Second Team All-Region, 2018 First Team All-Conference, and 2017 Big West All-Freshman Team.[2] The 2020 season in the Big West Conference was cancelled due to the COVID-19 pandemic.
In fall 2020, Trejo signed his first professional contract with the Los Angeles Force in the National Independent Soccer Association.[5] He started both of the team's Fall 2020 regular season matches and all four matches in the NISA Fall Championship, including the semifinal against Detroit City FC, and scored a goal against the New York Cosmos in the final group stage match on September 28.[6]
Trejo signed an MLS contract with Los Angeles FC on August 19, 2022.[8] Following the 2022 season, his contract option was declined by Los Angeles.[9]
Phoenix Rising
Trejo joined USL Championship side Phoenix Rising on January 26, 2023.[10][11] On August 15, 2023, Trejo was named USL Championship Player of the Week for Week 23 of the 2023 season after notching 2 goals and 2 assists during a 5–0 victory over El Paso Locomotive FC.[12] He became a free agent following Phoenix's 2023 season.[13]
Korona Kielce
On January 12, 2024, Trejo joined Polish Ekstraklasa side Korona Kielce on a two-and-a-half-year contract.[14] He made his league debut on February 12 as a substitute, scoring a volley from outside the box in injury time of a 2–1 win over ŁKS Łódź.[15]