[6] Lassiter also spent 5 years playing for Albion SC, a youth soccer club in San Diego, California.
He had 5 goals and 4 assists from 21 appearances during his first season for the Mustangs, earning a spot on the Big West Conference All-Freshman team.
Lassiter scored his first goal for Los Dos on April 11 to help Galaxy II to a 3–1 win over Sacramento Republic.
[2] Lassiter scored four times in the playoffs as Los Dos reached the final, where they fell 2–1 to the Rochester Rhinos.
[12] The 2015 season also saw Lassiter make one appearance for the Galaxy first team, coming on as a substitute in a 3–0 loss to the Houston Dynamo in an MLS match on July 25.
After being eliminated from the playoffs, Lassiter returned to the Galaxy first team and played in the final two games of the regular season.
On June 14, he scored his first goal for the Galaxy senior team to help LA defeat Orange County SC 3–1 in a U.S. Open Cup match.
He scored again on June 28 as the Galaxy defeated Sacramento Republic 2–0 to advance to the Open Cup Quarterfinals, where they fell 3–2 to the San Jose Earthquakes.
[10] Alajuelense opened the 2020 Clausura on January 12, 2020, beating Limón 4–1 win Lassiter scoring once and adding an assist.
Lassiter made four appearances and scored once in the playoffs as Alajuelense reached the final, where the fell to Saprissa 3–0 over two legs.
[23] He made his Dynamo debut on September 2, coming on as a substitute and scoring twice to give Houston a 3–0 win over Minnesota United.
[24] On October 7 Lassiter recorded his first assist for the Dynamo in a 2–0 win over Texas Derby rivals FC Dallas.
[30] On April 12, 2023, Lassiter was traded with teammate Bryce Duke to CF Montréal in exchange for Kamal Miller and $1.3 million in General Allocation Money.
[32] Born in Costa Rica and raised in the United States, Lassiter was eligible to play for both national teams.
[33] He made his senior debut for Costa Rica on March 22, 2019, in a friendly against Guatemala, coming on as a 63rd-minute substitute for Waylon Francis.
[35] Lassiter scored his first senior international goal on 12 July in a 3–1 over Guadeloupe in the Gold Cup group stage.
He made four appearances, scored one goal, and had one assist at the Gold Cup, helping Costa Rica reach the quarterfinals.