Judith Tebbutt, 56, from Bishop's Stortford, Hertfordshire, was flown to Nairobi after her family paid the pirates a ransom for her release.
She said she was "very relieved" and looking forward to seeing her "fantastic" son Oliver.
Her husband David, 58, was shot by a gang of six men at their remote holiday resort in Kiwayu, north of Lamu island.
'A good man'
Mrs Tebbutt, a social worker, said: "I'm just happy to be released and I'm looking forward to seeing my son who successfully secured my release. I don't know how he did it, but he did, which is great."
She said she was still coming to terms with her husband's death, which she only found out through contact with her son about two weeks after she was kidnapped.
AnalysisWithin hours of Judith Tebbutt's abduction last September the British government convened a crisis meeting known as a "Cobra".
Around 20 of these were held during the six months of her captivity, looking at all options for her release, peaceful and otherwise.
It was decided that those holding her were Somali pirates, purely after money, and not the extremist insurgency group, al-Shabab.
The British government doesn't pay ransoms so it was left to her son Oliver and family friends to hire a private security company to negotiate with her captors, while the family found the ransom money.
This contrasts with this month's failed hostage rescue by British Special Forces in Nigeria. Whitehall officials say that in that case the hostages were being held by an extremist group with links to al-Qaeda, the men's lives were in danger, and they had no choice but to intervene militarily.
"I am of course hugely relieved to at last be free, and overjoyed to be reunited with my son Ollie.
"This however is a time when my joy at being safe again is overwhelmed by my immense grief, shared by Ollie and the wider family, following David's passing in September last year.
"I hope that while I adjust to my freedom and the devastating loss of my husband, that I and my family will be allowed space, time and most of all privacy, to come to terms with the events of the last six months."
She landed in Nairobi on Wednesday where she will be looked after by officials from the British high commission before being flown back to the UK to be reunited with friends and relatives.
Prime Minister David Cameron's spokesman said the government would provide consular care in Nairobi. He said it did not pay ransoms and did not "facilitate concessions to hostage takers", but had met the family regularly to discuss the case.
It is understood Mrs Tebbutt's son Oliver is in the Kenyan capital to greet her.
Foreign Secretary William Hague said he was "delighted" Mrs Tebbutt had been released.
"Our immediate thoughts are with Judith's family and friends who have endured the ordeal of her captivity with great strength and dignity," he said.
Mrs Tebbutt's employer, Partnerships in Care, said staff and patients at Kneesworth House Hospital, which cares for adults with mental health issues, were "absolutely delighted" to hear of her release.
"We are all thinking of her and sending her our warmest regards. We hope to see her back at the hospital when she is ready," the statement added.
Mrs Tebbutt was seized on 11 September last year from Kiwayu Safari Village, a luxury resort on a deserted stretch of Kenyan coastline, comprised of thatched cottages on the beach.
The couple had arrived only the previous day and were the only guests.
She was taken away in a speedboat by Somali pirates, after her husband had been killed.
BBC security correspondent Frank Gardner said a private security company had secured her release, not British officials.
He said it was unclear how much money was involved, and revealing the amount was generally discouraged to avoid copy-cat gangs.
Paying the ransom was not illegal because it was not known to be going to a terrorist organisation, he added.
"She will now become the key witness in the ongoing murder investigation of her husband David," our correspondent said.
The BBC's East Africa correspondent Will Ross, in Nairobi, said the ransom had been paid in the last three days.
Rick Blears of Save Our Seafarers, the global anti-Somali piracy campaign, said that "any move at government level to ban the payment of ransoms to pirates, as US Secretary of State Hilary Clinton proposes, would have a massively detrimental effect and put the lives of hostages at grave risk".
Police in Kenya said six gunmen had burst into the Tebbutts' room last September and officers speculated that Mr Tebbutt may have tried to resist the gang.
He worked for publisher Faber & Faber and sat on the board of the Book Trade Charity, which supports those in the book trade.
The Kenyan government said at the time of the kidnap that it believed the al-Shabab Islamist group, which has since merged with al-Qaeda, was behind the murder and kidnap. The group denied the allegation.
Rachel Chandler, who was held with her husband Paul for 13 months by pirates in Somalia, said: "My feeling is one of relief and happiness for Judith Tebbutt and her family, that finally she is free."
Mr Chandler added: "I hope she will have an opportunity to pick up the pieces of her life, and deal with the loss she has had."
Last year, two men appeared in court in connection with the attack, with both denying the charges.
One of them, Ali Babitu Kololo, told the court he had been forced at gunpoint to lead a group of men to the hotel and had not been a willing accomplice.