Sebastian Francisco Lletget was born in San Francisco in 1992 to parents of Argentine descent. Lletget played for Santa Clara Sporting in northern California and then joined the United States U-17 program.

In 2009, he was spotted by scouts from West Ham United and moved to England to play in their academy. He signed his first professional contract in September 2010 as an 18-year-old.

Lletget played for the Hammers academy and youth teams while also participating in pre-season friendlies. He was an unused sub for four Premier League games in 2012/13 but was sidelined with mononucleosis.

The player signed a new contract with West Ham in 2013 and made his debut on January 5, 2014, as the Hammers lost 5-0 to Nottingham Forest in the third round of the FA Cup.

LA Galaxy acquired Lletget in May 2015 after sending $50,000 in Allocation Money to the New England Revolution. They signed Lletget on May 8 and he debuted a week later against Orlando City SC as a substitute.

After playing one USL game with LA Galaxy II, Lletget returned to MLS and made his first start on June 13, scoring the opening goal against Columbus Crew. Lletget scored in the US Open Cup and added goals in the next two MLS games for a four-game scoring streak. He finished his rookie MLS season with seven league goals and two assists in 20 games. In the 2015 MLS playoffs, Lletget scored Galaxy's first goal in a 3-2 knockout round loss to Seattle Sounders.

In 2016, Lletget played in 31 games for the Galaxy with 23 starts. He provided eight assists but scored just one goal in league play. Lletget was among the top seven on the roster with 2,057 minutes played and he was named in the Week 10 MLS Team of the Week.

In the US Open Cup, he scored twice in extra time as the Galaxy beat La Maquina FC in the fourth round and added two more goals against the Seattle Sounders in a quarter-final win. LA Galaxy went on to lose in the semi-finals. Lletget also started all three MLS playoff games, but the Galaxy were eliminated in the Western Conference semi-finals.

After playing all 90 minutes in the first three games of the 2017 season, Lletget missed the rest of the year after suffering a Lisfranc injury with the US men's national team.

He returned to action as a sub on March 4, 2018, against Portland and started for the first time in a year on March 11 against NYCFC. Three weeks later, Lletget scored his first MLS goal in nearly two years with the Galaxy's first against LAFC to start a 4-3 comeback win. He scored as a sub on August 11 against Minnesota and scored again three days later against Colorado Rapids. He finished the regular season with three goals and two assists in 28 games.

Lletget opened the 2019 campaign with one goal in his first two starts but didn't score again for the next 26 games. He helped out with four assists in that span and finally ended his scoring drought with a brace against Sporting KC on September 15. He finished the 2019 regular season with three goals and five assists in 29 games. In the 2019 playoffs, Lletget scored the opening goal against Minnesota United FC in the first round. Galaxy won 2-1 but fell to LAFC in the Western Conference semi-finals.

Lletget played in both of the Galaxy's 2020 games before the COVID-19 shutdown. In the MLS Is Back Tournament in Orlando, he started all three group stage games, but the Galaxy went home with just one point.

Back in the resumed MLS season, he scored in consecutive games in August while also adding an assist against San Jose. He provided another assist against Portland and extended his goal/assist streak to four games with a brace against LAFC on September 6. Lletget again scored in consecutive games on September 27 and October 3. He also provided assists in consecutive games on October 28 and November 1 as the Galaxy were eliminated from playoff consideration.

Lletget played in 21 games in 2020 with 19 starts. His six goals were the most since 2015 and went alongside four assists. He set new career highs with 37 shots, 18 shots on target and 20 fouls won.

As a youngster, Lletget played for the United States at the U-17, U-20 and U-23 levels. He was called up to the senior national team for the first time in January 2017. His debut came against Serbia in a friendly on January 29 and he also started against Jamaica a month later.

Lletget's first national team goal came against Honduras in a World Cup qualifier on March 24, 2017, in San Jose, California, near where he grew up. He suffered a Lisfranc injury in the same game and missed the rest of the 2017 season for club and country. The injury cost him a spot in the 2017 Gold Cup team.

He returned to the USMNT on November 15, 2018, coming off the bench for 14 minutes in a 3-0 loss at Wembley Stadium against England. Five days later, he came off the bench again in a 1-0 defeat against Italy in Belgium.

Back in his home area of San Jose, Lletget scored the game winner in a friendly against Costa Rica while also adding an assist in February 2019. He played in eight games in 2019 with one goal scored.

The US were limited to just three games, all friendlies, in 2020. Lletget played all 90 minutes against Costa Rica in February and he played 87 minutes against Wales in November. He scored his third international goal on November 16 against Panama in Austria. Lletget came off the bench in the 76th minute and scored 11 minutes later to give the US a 5-2 lead.

Overall, he has played 16 games for the US with three goals scored and one assist.

Lletget's parents Sara and Francisco are Argentine and he also holds an Italian passport through his grandfather. Lletget is in a long-term relationship with Mexican-American pop star Becky G. They confirmed their relationship in 2016.

Becky G has opened tours for Demi Lovato, Katy Perry and Enrique Iglesias. She headlined the Mala Santa Tour in 2019 and won the American Music Award for Favorite Female Latin Artist in 2020.

Lletget is estimated to be worth in the region of $1million. His 2020 base salary from the Galaxy is $300,000 per year and he is guaranteed $312,000 in compensation. His 2019 salary was the same after he had signed an extension, earning a boost from a salary of $240,000 in 2018.