Overeem was born in Hounslow, London, England, to a Jamaican father and a Dutch mother, whose maternal great-grandmother was an illegitimate child of King William III of the Netherlands.

[9] As a child, Overeem started training martial arts with his older brother Valentijn in order to defend himself from bullying.

[10][11] Alistair initially did not like the sport, as he was routinely beaten by more experienced students, but he changed his mind when he met Bas Rutten and Joop Kasteel, after which he fully dedicated himself to training.

[10][11] Overeem had his first MMA professional fight at 19 years old, defeating Ricardo Fyeet by submission on 24 October 1999 (1999-10-24) at It's Showtime, the first event of the eponymously named promotion.

Overeem moved on to fight Igor Vovchanchyn in the quarterfinals at Pride Critical Countdown 2005 and won via submission in under two minutes.

On 9 June 2006, Overeem travelled to San Jose, California, to win a rematch with Vitor Belfort by decision at Strikeforce: Revenge.

He then suffered another KO loss to Maurício Rua at Pride 33, but returned in June 2007 with a submission win over Michael Knaap at K-1 Grand Prix in Amsterdam (despite K-1 being a kickboxing promotion, the bout was fought under MMA rules).

On 16 November 2007, Overeem defeated Paul Buentello for the vacant Strikeforce Heavyweight Championship by submission due to knee strikes.

[13] The bout was stopped about halfway through the first round and declared a no-contest, after Overeem landed multiple knees to the groin of Cro Cop.

Overeem was rumoured to be making his first title defence against Brett Rogers on 6 June at Strikeforce: Lawler vs. Shields.

[14] According to Golden Glory manager Bas Boon, Overeem had suffered a hand infection following a nightclub brawl early May 2009.

[19] The majority of Alistair Overeem's pre-fight training camp took place at the newly opened Golden Glory Gym in Pattaya, Thailand, where "The Demolition Man" concentrated on the further improvement of his Muay Thai skills.

[22] In early 2011, Overeem was named as one of eight men that will take part of the Strikeforce Heavyweight Grand Prix, alongside Fabrício Werdum, Sergei Kharitonov, Brett Rogers, Josh Barnett, Andrei Arlovski, Antônio Silva and Fedor Emelianenko.

Overeem went on to state that he felt the September date was too soon for him to return, and that he needed time to rest and heal after his 18 June quarterfinal decision win over Fabrício Werdum.

[25] Subsequently, on 29 July, it was announced that Overeem had been released from the Strikeforce organization as Zuffa exercised their right to eliminate the one remaining fight on his contract.

Team Golden Glory leader Bas Boon stated he has since changed the policy, and that Alistair Overeem could come to the UFC under an exclusive deal if the right terms are offered, which he later received.

Overeem hurt Lesnar multiple times early on with knees to the body, and went on to finish the fight with a kick to the liver and subsequent punches at 2:26 of the first round.

However, on 4 April 2012, Overeem was revealed to have failed his pre-fight drug test by the Nevada State Athletic Commission (NSAC).

On Friday 20 April 2012 UFC President Dana White confirmed that Overeem had been removed from his fight with dos Santos and replaced by Frank Mir.

"[35] Overeem's lawyer filed a continuation request for additional time to gather support for his recent claim, which was voted on by the NSAC on 24 April 2012.

At the K-1 World Grand Prix 2009 Final, Overeem knocked out the Kyokushin karate champion, Ewerton Teixeira, with a knee strike in the first round, but lost to Badr Hari via TKO in the semifinals.

Over a decade removed from his previous kickboxing bouts, news surfaced on 8 June 2021, that Overeem had signed a multi-fight contract with Glory.

Overeem was scheduled to headline the first Wrestling Entertainment Series (WES) event in England against WWE wrestler Braun Strowman.

[97] During the 2023 Dutch general election, Overeem participated for the last candidate on the party list of Belang van Nederland (BVNL).

[101] A kickboxer of the Dutch school of Muay Thai decorated in K-1 world tournaments,[102] Overeem favours kicks to the body and legs and knee strikes from the clinch,[103] which have been described as "devastating.

[103] On the grappling field, Overeem is universally known for his usage of the guillotine choke,[101][102][103] a simple technique he uses to great effect thanks to his strength and height.

Alistair started arguing with a bouncer when he found himself without coins to pay the toilet woman and was asked to leave by five other security workers.

The incident ended with five bouncers having to go urgently to the hospital and Alistair being forced to turn down a title fight due to a hand injury gained in the brawl.

Not arrested but summoned to court, Overeem was charged with misdemeanor battery and faced a maximum of six months in the Clark County Detention Center and a fine up to $1,000.

[108] On 28 March 2012, Overeem was given a 90-day county jail sentence that would be suspended depending on the completion of 50 hours of community service and anger management.