He began playing football as a child, initially as a form of escapism from the Sierra Leone Civil War, in which he witnessed executions in his home city of Kenema, infiltrated early by rebels.

[4] At age 16, Kamara and his family migrated to the United States through a refugee program, initially moving to Maryland,[5] before settling with relatives in Hawthorne, California, near Los Angeles.

[4] Kamara played college soccer at California State University, Dominguez Hills, where, in his second year, he was named third-team All-American.

[8] Kamara also spent two seasons with Orange County Blue Star in the USL Premier Development League.

On 24 July 2008, Kamara was traded to Houston Dynamo for a first-round pick in the 2009 MLS SuperDraft and allocation money.

[14] Norwich City announced on 5 February that Kamara was granted a visa, making him legally eligible to play for the club.

On the next matchday, 23 February, he came on as a 58th-minute substitute, and 19 minutes later, scored his first goal for Norwich, equalising 1–1 against Everton with a powerful header at Carrow Road, in an eventual 2–1 victory.

[24] After parting ways with Middlesbrough, Kamara had a successful trial with recently promoted Championship side Wolverhampton Wanderers.

However, his work permit renewal application was declined after Sierra Leone fell to 75th place in the FIFA World Rankings and Wolverhampton shifted its focus to signing Yannick Sagbo instead.

[25] In late August 2014, it was revealed that Kamara could return to MLS but would have to go through the league's allocation ranking system since Kansas City received a transfer fee for the player.

1 spot in the allocation ranking was held by Columbus Crew SC, the club which initially drafted Kamara in 2006.

[34] Due to an injury sustained in training, however, it was initially uncertain whether Kamara would be able to feature in the 2015 MLS Cup Final against the Portland Timbers;[35] nevertheless, he played all 90 minutes of the match, scoring the only goal for Columbus in a 2–1 defeat.

[50] On 11 December 2018, Kamara was selected by FC Cincinnati in the 2018 MLS Expansion Draft and immediately traded to Colorado Rapids in exchange for an international roster slot for the 2019 season.

On 18 September 2020, Colorado traded Kamara to Minnesota United for allocation money and a 2022 second round MLS SuperDraft pick.

[54] In February 2022, Kamara returned to North America, joining CF Montréal on a one-year deal, with an option for 2023.

[55] He made his debut for his new club on 23 February against Santos Laguna of Liga MX in a 2022 CONCACAF Champions League match.

[61] On 8 April, he scored a first-half brace against his former team Minnesota United FC to help his side to a 2–1 win, thus ending the Loon's 5-match unbeaten start.

[66] In the following match on 22 June, he scored his 146th MLS goal against San Jose Earthquakes to pass Donovan as the second-highest goalscorer in league history, only behind Chris Wondolowski (171).

[67] On 4 July, he opened the scoring against La Galaxy in an eventual 2–1 win at the Rose Bowl, and in doing so at the age of 39 years and 10 months, he became the oldest scorer in El Tráfico history.

Thus, he contributed decisively to helping Los Angeles reach the Leagues Cup Final for the first time in its history, which they lost 3–1 to Columbus Crew.

[76] He came out of retirement for a second time in November 2024 at the age of 40 to play against Chad in Sierra Leone's penultimate 2025 Africa Cup of Nations qualification game.