West Ham United Lletget signed his first professional contract with West Ham in September 2010. He played in pre-season friendly games and was an unused substitute for four
Premier League games for the
2012–13 season.
LA Galaxy On May 8, 2015, Lletget signed for
LA Galaxy. Lletget debuted for the Galaxy on May 17, 2015, in the 4–0 loss against
Orlando City SC when he replaced
Mika Väyrynen in the 69th minute. On May 30, Lletget started for
USL affiliate club
LA Galaxy II against Colorado Springs that ended in a 2–1 victory for the Galaxy. Lletget made his first start with the first team on June 13 against
Columbus Crew, in a match where he scored the opening goal of the game, which was also his first for the Galaxy. Lletget scored again 4 days later in an
U.S. Open Cup match against PSA Elite that ended in a 6–1 victory. On June 20, Lletget scored for the third match in a row in a 5–1 victory over the
Philadelphia Union. Lletget scored for the fourth match in a row in a 5–0 victory over
Portland Timbers on June 24. In the 2016 season, Lletget scored a brace on June 14, 2016, against
UPSL side
La Máquina FC in extra-time of the Fourth Round of the
U.S. Open Cup. Lletget again scored a late brace in the U.S. Open Cup, helping the Galaxy move to the semi-finals after 4–2 victory over
Seattle Sounders FC. After spending the
2017 LA Galaxy season recovering from an injury incurred while playing for the
United States national team, Sebastian Lleget opened the scoring for the Galaxy in the 61st minute of the inaugural
El Tráfico, adding his first goal since his return. Lletget was then subbed out for the debut of
Zlatan Ibrahimović.
New England Revolution On December 14, 2021, Lletget was acquired by reigning
Supporters' Shield holders
New England Revolution. He got his first start and goal in the opening match of the season, a 2–2 draw against
Portland Timbers. On March 9, 2022, Lleget scored the game winner in the Revolution's quarter final
2022 CONCACAF Champions League matchup against
Pumas UNAM. He scored his second MLS league goal on May 21, 2022, in the Revolution's 3–2 win against
FC Cincinnati.
FC Dallas On August 3, 2022, FC Dallas acquired Lletget from the Revolution in exchange for $600,000 in
allocation money. The deal would reportedly see the Revolution receive $300,000 in allocation money in 2022 and an additional $300,000 in 2023. On November 20, 2025, the team announced that they declined his contract option and that he would become available for MLS free agency.
Monterey Bay FC On March 12, 2026,
USL Championship side
Monterey Bay FC announced the signing of Lletget on a free transfer. ==International career==