Kei Kamara joined Norwich City on loan from MLS side Sportng Kansas City on 30th January 2013. 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.

Kamara was born and raised in Kenema, Sierra Leone's third largest city. As a teenager, he emigrated to the United States to escape the civil war in that country, and settled with relatives in Hawthorne, California, near Los Angeles.

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 then chosen by Columbus Crew ninth overall at the 2006 MLS SuperDraft. Prior to the 2008 season, he was traded to the San Jose Earthquakes in exchange for Brian Carroll. On July 24, 2008 Kamara was traded to the Houston Dynamo for a draft pick and allocation money. On September 30, 2008 Kamara scored two goals for MLS champion Houston Dynamo against Mexican club UNAM Pumas in a CONCACAF Champions League match. On September 15, 2009 Kamara was traded to Sporting Kansas City for Abe Thompson and allocation money.

Following several productive seasons with Kansas City which saw him become one of the most imposing wingers in the league, he trialled with Stoke City in December 2012 before joining Norwich.

At the time of his switch to Norwich, he had been capped 14 times by Sierra Leone, scoring twice.

Kamara made his debut for Norwich on 9th February 2013 against Fulham, coming on as a substitute with four minutes remaining. He scored his first Norwich goal in the game against Everton which City won 2-1 on February 23rd 2013. Kamara came on for Luciana Becchio on 58 minutes with City 1-0 down. Kamara scored the equaliser on 84 minutes heading home a Robert Snodgrass corner before Grant Holt scored the winner in injury time.

He was called up on 13th March 2013 to the Sierre Leone international squad for the World Cup qualifier against Tunisia at the end of March 2013.

Kamara returned to the USA on 4th May 2013 following the 2-1 home defeat to Aston Villa in which he came on as substitute. He had played 11 games for Norwich, scoring one goal.

Kamara left Sporting Kansas City to join Middlesbrough on Transfer Deadline Day (2nd September 2013) for a fee of £800,000.

Kamara left Boro on 28th August 2014 by mutual consent. Kamara then had a successful trial with Wolverhampton Wanderers of the Championship. However, Kamara's 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.

On October 7th 2014, he signed for MLS side Columbus Crew but would officially be added to the roster on January 1st 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 2014 Major League Soccer season ended

On 12th May 2016, Columbus traded Kamara to the New England Revolution for a record fee. One source told ESPN that the amount of targeted and general allocation money involved is in excess of $300,000. Other reports said that New England gave up a war chest full of roster tools, including around $500,000 in allocation money, reportedly a record sum in a league with a $3.67 million soft salary cap; a first-round pick in the 2017 draft; a second-round pick in 2018; an international roster spot for the rest of this season, and a percentage of any future transfer money paid for Kamara if he leaves MLS to play abroad. This was despite Kamara scoring 22 goals the season before and with Columbus struggling in their division. However Kamara had a public spat with the Crew over who should take a penalty and lashed out at teammate Federico Higuin, earning him a one game suspension and fine. His deal with New England will see him earn $1million per season.

On 10th December 2017, Kamara became Vancouver Whitecap's second signing of the 2017-18 winter close season. Kamara will officially be added to the club's roster pending receipt of his international transfer certificate, Canadian work permit, and visa. He will occupy a Designated Player roster slot and is a domestic player as an American citizen. "Kei is a player that brings goals and experience to the table," said Whitecaps FC head coach Carl Robinson. "He's a big body and he thrives off crosses. Most importantly, he's a good character and he's driven to win. We firmly believe that Kei can help lead this team into exciting times next year." He was signed in exchange for Vancouver's 2019 natural first round SuperDraft pick and a conditional 2020 second round SuperDraft selection. He had scored 12 goals for New England. .

At the end of the 2018 season, Kamara left the Whitecaps. He had scored 17 goals in 31 games. On 11th December 2018, he was signed by FC Cincinatti in the 2018 MLS Expansion Draft and immediately traded to the Colorado Rapids.

On 19th September 2020 Kamara moved again, signing for Minnesota United, his eighth MLS club. The deal included the Rapids receiving $150,000 allocation money and a second round draft pick. He made his Minnesota debut on 24th September 2020 in a 2-1 win over Columbus Crew. He became the first player to score for eight different teams in Major League Soccer when he netted from the penalty spot for Minnesota United against FC Cincinatti on 3rd October 2020. This was Kamara’s 130th MLS goal, taking him to fifth on their all time goalscorers’ list. He left Minnesota in December 2020.

At the end of July 2021 he signed for Finnish side HIFK Helsinki until the end of the 2021 season. "I've officially signed a deal with HIFK. Yep, it's official," Kamara said in a Twitter post. "I'm so grateful. Here I am in beautiful Finland, I'm out here. The most historic club in the country, that's who I'm with. I'm super excited."

On leaving HIFK, he competed for Sierra Leone at the 2022 Africa Cup of Nations. On 18th February 2022 he joined CF Montreal of the MLS for the 2022 MLS season with an option year for 2023.

On 24th February 2023 he was traded to the Chicago Fire in return for $250,000 in general allocation money split over the next two seasons. This could increase based upon Kamara's performances.

On 29th March 2024 he joined Los Angeles FC for the 2024 season.