During his time with the Rotterdam-based club, he scored 35 goals in 67 league appearances, also winning the Johan Cruijff Award in 2005 as the most promising young talent of the season.

Kalou, together with Dirk Kuyt, were affectionately known as "K2" by Feyenoord fans and the Dutch media, a play on the words of K3, a Belgian pop band.

Chelsea manager José Mourinho praised the young Ivorian as being hardworking, versatile, eager to improve and unafraid of the physical side of the game.

Kalou scored his first goals for Chelsea in a reserve game against Portsmouth, bagging a hat-trick and earning the match ball in a 5–0 thrashing.

He made his competitive debut in the 2006 FA Community Shield,[8] and then scored his first senior goal for Chelsea in a two-goal win over Blackburn Rovers in the third round of the League Cup.

Kalou did not feature much under Avram Grant's successor, Luiz Felipe Scolari, except during pre-season, because of his participation in the 2008 Beijing Olympics as part of the Ivory Coast national under-23 football team.

[11] On 14 January 2009, Kalou scored a tap in against Southend United in the third round of the FA Cup as Chelsea ran out 4–1 winners in the replay.

[12] Again he scored two goals against Middlesbrough, this time at Stamford Bridge on 28 January, a 2–0 Chelsea win, becoming a favourite under interim manager Guus Hiddink.

He scored his first goal of the 2009–10 season against Queens Park Rangers at Stamford Bridge in the third round of the League Cup, earning his team a 1–0 victory.

[19] He celebrated his new contract with an brace against Atlético Madrid in the Champions League, helping Chelsea maintain their perfect record in the competition.

His shot from outside the box was tame but an error from Inter goalkeeper Júlio César allowed the ball to find the back of the net in a 2–1 loss.

[20] In Chelsea's 7–0 win over Stoke City on 25 April, Kalou netted a hat-trick, though his second goal of the day had been the source of controversy, as it was very close to being a two-footed tackle.

He appeared as a substitute in the 6–0 drubbing of Wigan at the DW Stadium to score two goals, both assisted by fellow Ivorian Didier Drogba.

[25] Nonetheless, he was included in the starting lineup for their next game against West Bromwich Albion, scoring a goal in Chelsea's 3–1 win on 16 April.

[27] For Chelsea's crucial game against Tottenham Hotspur, Kalou was not included in the starting lineup, as Carlo Ancelotti preferred a striking partnership of Drogba and Torres.

[35] Following the appointment of Roberto Di Matteo as interim manager, Kalou fought his way back into the first-team squad; under André Villas-Boas, he was frozen out, only making four starts in over seven months.

Kalou then netted a vital away goal in the first leg of their Champions League quarter-final clash with Benfica after some great work from strike partner Fernando Torres.

[40][41] On 1 July 2012, Chelsea officially announced that Kalou's contract had come to an end along with José Bosingwa, after spending six years at the London side.

After his release, Kalou was linked with numerous clubs across Europe, including the likes of Schalke 04,[42] Liverpool,[43] Galatasaray,[44] Newcastle United[45] and Arsenal.

[53] In his first full season at the Berlin-based club, he played 27 times in the league, scoring six goals and helping Hertha avoid relegation.

[61] With his goal on 30 January 2016 at Werder Bremen, Kalou curiously became the first player to have double-digit goal-scoring seasons in the Premier League, Ligue 1, and the Bundesliga.

Kalou played in Hertha's brief Europa League stay against Brøndby IF, as well as their first round Pokal match against Jahn Regensburg, but did not feature in the Bundesliga initially for Pál Dárdai.

[64] On 4 November, Kalou emphatically ended his scoreless start to the season, picking up a hat trick against Borussia Mönchengladbach in a 3–0 victory, moving his side temporarily into third place.

Kalou greeted players and coaches, who were unaware of the live broadcast, with a handshake, disregarded social distancing regulations and filmed a conversation about the Hertha professionals' salary cuts.

[78] During the process, Dutch insurance company Centraal Beheer made a television advertisement in which Kalou represents the Netherlands' rivals Germany against them.

[86] That June, he went to the 2010 FIFA World Cup in South Africa and scored as a substitute in a 3–0 win over North Korea in the final group game, though the Elephants were eliminated.

[87] On 26 January 2012, Kalou scored a goal in Ivory Coast's 2–0 win over Burkina Faso, helping his side progress into the quarterfinals of the Africa Cup of Nations.

[89][90] In the 2015 Africa Cup of Nations, he came on as a substitute for Siaka Tiéné for the last four minutes of extra time in the final, and scored in a 9–8 shootout win over Ghana.