Lewthwaite was born to parents Andrew and Elizabeth Christine (née Allen) Lewthwaite in Banbridge, County Down in 1983. Her father is a former British Army soldier who served in the 9th/12th Royal Lancers and had met her mother while he was stationed in Northern Ireland in the 1970s. Following her birth the family lived for a short period in Northern Ireland, where her father worked as a lorry driver, before settling in Aylesbury, England. She attended Elmhurst middle school and The Grange secondary school in Aylesbury. She studied for a politics and religion degree at the School of Oriental and African Studies, University of London, but dropped out.
Lewthwaite's parents separated in 1994, and friends later reported that she was "badly affected by the break-up" and "sought solace from Muslim neighbours who she believed had a stronger family network." Raised as a Christian, by the age of 17 she had converted to Islam. She adopted the Muslim name Sherafiyah at the time of her conversion. She arranged to meet Germaine Lindsay at a Stop the War march in Hyde Park, London; they subsequently married in Aylesbury on 30 October 2002, using the Islamic names Asmantara and Jamal. Lewthwaite's parents, who "never came to terms with their daughter's conversion", refused to attend the ceremony.
In September 2005, Lewthwaite was widely criticised for selling her story, in which she portrayed herself as a victim and her husband as a "relatively recent [convert]" who had been "tricked into his actions by extremists", to tabloid newspaper The Sun for £30,000. The Independent reported that Lewthwaite's account conflicted with evidence from Lindsay's sister that he had actually converted to Islam aged 15, and said that families of the victims were "unconvinced by her portrait of the bomber", while her "attempts to share the blame with others obscured the murder of innocent commuters". The Yorkshire Post said: "For very good and obvious reasons, there is a law against any Criminal profiting from his illegal activities by selling his story to a newspaper. And while the letter of the law has not been broken on this occasion—Ms Lewthwaite is not a criminal—its spirit has clearly been breached."
Kenyan police revealed Grant was already known to them after being arrested in the Dadaab refugee camp in northern Kenya in 2008 while attempting to travel to Somalia disguised as a woman wearing a burqa. He and two other men, also wearing burqas, were allegedly travelling to meet an Al-Qaeda leader. Six hours after their arrest, a group of 20 Al-Shabaab fighters freed all three men after storming the Dadajbula police post where they were being held. One of the other two men is believed to have been Saleh Ali Saleh Nabhan, later killed by US Navy SEALs in Operation Celestial Balance, an attack against an Al-Qaeda training camp in Somalia in 2009. A woman who was with the group at the time of their arrest but not detained was subsequently identified by Kenyan police as Lewthwaite.
Lewthwaite gave birth to her fourth child, a daughter, at a private birthing centre in Johannesburg in July 2010. The Daily Telegraph reported that she registered with the clinic late into her pregnancy, and under the name Asmaa Shahidah Bint-Andrews. She gave birth after attending four prenatal appointments. Anti-terrorism investigators told the newspaper the child's father is believed to be Abdi Wahid, a former Kenyan naval officer who defected to al-Shabaab.
Kenyan police revealed that when Grant was arrested in December 2011, the group was only days away from committing an attack. The target was either "a hotel in Mombasa or a shopping mall in Nairobi." Grant was also alleged to have voluntarily given up the identity of Lewthwaite, telling police officers: "There is someone much bigger you really want; she is the financier." In March 2012, following a request from the Kenyan authorities, Interpol issued an international alert requesting the arrest and extradition of Lewthwaite.
In July 2012, she was named as one of the suspects involved in a 24 June 2012 grenade attack on the Jericho bar in Mombasa. The attack took place during a Euro 2012 football match between England and Italy. Kenyan police said a woman matching Lewthwaite's description was seen near the bar shortly before the attack in which three people were killed and 25 injured. Police said: "We suspect Samantha Lewthwaite was actively involved in the terrorist attack on the club."
Lewthwaite's name was linked with the September 2013 attack claimed by Al-Shabaab on the Westgate shopping mall in Nairobi. Despite intense media speculation, these reports were viewed with caution by UK government officials, and there was no confirmation of Lewthwaite's involvement as an attacker, organiser or fundraiser. Al-Shabaab itself claimed no women played a role in the attack. In a Twitter post the group said, "We have an adequate number of young men who are fully committed and we do not employ our sisters in such military operations."
On 13 November 2014 the commander of the battalion reportedly rejected the reports as false: "We don't have any White Widows, and nor could we. [The Russians are] trying to drive a wedge between us and our British allies."