Kamara was born and raised in Kenema, Sierra Leone's third largest city, and began his career playing for Kallon F.C. from 2001-2003. At age 16, Kamara and his family migrated to the United States through a refugee program, and settled with relatives in Hawthorne, California, near Los Angeles. He became an American citizen.
Kamara played college soccer at California State University, Dominguez Hills, where, in his sophomore season, he was named third-team All-American. Kamara also spent two seasons with Orange County Blue Star in the USL Premier Development League.
Kamara was chosen by Columbus Crew ninth overall at the 2006 MLS SuperDraft. He spent two seasons there, making 36 appearances and scoring 5 goals.
Prior to the 2008 season, he was traded to San Jose Earthquakes in exchange for Brian Carroll. However, he only spent the 2008 season there, making 12 appearances and scoring 2 goals.
On 24 July 2008, Kamara was traded to Houston Dynamo for a first-round pick in the 2009 MLS SuperDraft and allocation money. On 30 September 2008, Kamara scored two goals for MLS champion Houston Dynamo against Mexican club UNAM Pumas in a CONCACAF Champions League match.
On 15 September 2009, Kamara was traded to Sporting Kansas City for Abe Thompson and allocation money. Kamara had several productive seasons with Kansas City which saw him become one of the most imposing wingers in the league.
Kamara was a transfer target of Norwich City of the English Premier League during the winter 2013 transfer window, following an unsuccessful trial at Stoke City earlier in the off-season. It was announced on 30 January 2013 that Kamara was signed on loan by Norwich for the remainder of the 2012–13 Premier League season. Norwich had the option for a pre-negotiated permanent transfer or to return Kamara to Sporting at the end of the season, in which case Kamara would miss the first 10 matches of the MLS season. As part of the deal, Kamara's contract with Major League Soccer was extended. Norwich City announced on 5 February that Kamara was granted a visa, making him legally eligible to play for the club. He made his league debut for Norwich as an 86th-minute substitute in a 0–0 draw with Fulham four days later. 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. Kamara assisted team-mate Wes Hoolahan in the 1–1 draw against Sunderland on 17 March, as his header past the goalkeeper was deflected into the net by Hoolahan who was standing on the goal-line. Kamara later claimed the only reason he did not claim the goal as his own was that it was St. Patrick's Day, and he and Irishman Hoolahan had an Irish dance celebration planned.
Kamara quickly became a fan favourite of the Norwich support for his enthusiastic and flamboyant style of play, with some fans composing songs to recognise their new-found hero. Despite this, Kamara did not score any more in a Norwich shirt and Norwich declined the option of taking him on a permanent transfer. On 4 May 2013, Norwich City confirmed the loan deal had ended and he would be returning to Sporting Kansas City immediately.
On 2 September 2013, it was confirmed that Kamara had signed for Middlesbrough of the Championship on a two-year contract for a fee of £900,000. He made his debut for the club twelve days later, coming on as a 63rd-minute substitute in a 1-3 loss to Ipswich Town. On 17 September, Kamara made his first start and scored his first goal for Boro in a 2–2 draw at Nottingham Forest. He followed up his first goal against Forest by scoring on his home debut, a 3–3 draw against Bournemouth four days later. He then scored on 5 October in a 4–1 win at home to Yeovil Town, and twenty days later in a 4–0 home win at over Doncaster Rovers. On 28 August 2014, it was announced that Middlesbrough had parted company with Kamara as his contract had been cancelled by mutual consent.
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.
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. At the time, the No. 1 spot in the allocation ranking was held by Columbus Crew SC, the club which initially drafted Kamara in 2006. On 7 October 2014 it was confirmed that Kamara had signed for Columbus and would officially be added to the roster 1 January 2015. Because he was released by Middlesbrough after the 2014 MLS roster freeze, he would not be able to appear for the Crew until the 2015 Major League Soccer season.
The following season, he helped the Crew to the play-offs by finishing second in the Eastern Conference, and finished the 2015 MLS season as the joint top-scorer of the league, alongside Toronto FC's Sebastian Giovinco, with 22 goals in 33 games, although Giovinco claimed the MLS Golden Boot due to having provided more assists throughout the season (16 compared to Kamara's 8), which was the tie-breaking criterion. In November 2015, he was named one of the three finalists for both the 2015 MLS Landon Donovan MVP Award and the 2015 Advocare MLS WORKS Humanitarian of the Year Award; he was awarded the MLS Humanitarian of the Year Award on 5 November, although he missed out on the MVP Award, which went to Giovinco. On 8 November, he scored two goals in a 3–1 win over Montreal Impact in the Eastern Conference semi-finals of the MLS Playoffs, including the decisive goal in extra-time, which allowed Columbus Crew to advance to the Eastern Conference finals. On 22 November, he scored in the first leg of the 2015 MLS Playoff Eastern Conference Finals, a 2–0 win over the New York Red Bulls, which allowed Columbus to claim the 2015 Eastern Conference Championship 2–1 on aggregate and advance to the MLS Cup Final. 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; nevertheless, he played all 90 minutes of the match, scoring the only goal for Columbus in a 2–1 defeat. With 4 goals, Kamara was the top-goalscorer in the 2015 MLS Playoffs, bringing his total seasonal tally to 26 goals although he missed out on the MLS Cup MVP Award, which went to Portland's Diego Valeri.
After being signed to a Designated Player contract, Columbus got off to a rough start in the 2016 season. Kamara scored five goals in his first nine games including a brace against Montreal on May 7, 2016 where Columbus had the lead. An argument arose between Federico Higuain and Kamara over a penalty kick, with Kei potentially getting his first hat trick in his career, however Higuain wanted to take it himself. The game ended in a 4-4 draw. After the game, Kamara publicly called Higuain out in an interview after the game. As a result, he was suspended by manager Gregg Berhalter for the next game.
On May 12, 2016, just 5 days after the penalty kick incident with Higuain, Kamara was traded to New England Revolution in a blockbuster deal which saw Columbus receive a first-round pick in the 2017 MLS SuperDraft, a second-round pick in the 2018 MLS SuperDraft, an international roster slot for the 2016 season, general allocation money, targeted allocation money, and a percentage of any fee received should New England transfer Kamara to a club outside of MLS.
He scored his first goal for New England in a 2-1 win over the New York Cosmos.
On September 2nd 2017, he scored his first MLS hat trick against Orlando City in a 4-0 win.
Kamara made his debut for the Sierra Leone national football team, commonly referred to as the Leone Stars, on 5 September 2010, during a 1–1 draw with Egypt in Sierra Leone's first group stage match in qualification for the 2012 Africa Cup of Nations. Kamara has since formed a regular partnership up front with fellow Kenema native Mohamed Kallon in the Leone Stars attack.Scores and results list Sierra Leone's goal tally first.
Kamara earned his U.S. citizenship in 2006.Sporting Kansas City
Eastern Conference (Regular Season) (2): 2011, 2012
Lamar Hunt U.S. Open Cup: 2012
Columbus Crew SC
Eastern Conference (Playoffs): 2015
MLS Fair Play Award: 2007
Sierra Leonean Player of the Year: 2012
MLS Joint Top-scorer: 2015 (22 goals, alongside Sebastian Giovinco)
MLS All-Star: 2015
MLS Works Humanitarian of the Year: 2015
MLS Best XI: 2015
MLS Cup Playoffs Top-scorer: 2015