Mavis Paterson, who lives near Stranraer in Dumfries and Galloway, set out on May 30 from Land's End and reached John O'Groats on Saturday.
Mrs Paterson, who took up the challenge in memory of her three children, who died within four years of one another, raised more than £50,000 for Macmillan Cancer Support - smashing her £20,000 target.
Guinness World Records confirmed before the trip that she would become the oldest woman to cycle the length of Britain if she successfully completed the distance.
Mrs Paterson was joined on the challenge by cycling companion Heather Curley, 55.
A post on the team's Facebook page said: "It's hard to find the words now. After three weeks of an utterly unforgettable journey. We did it.
"So very, very grateful for all the support, the fun, friendships, cyclists who joined for a few miles, vehicle drivers - especially for their tea-making skills, campsites, tearooms, gifts of home baking, gifts of cycling gear, teacakes and caramel wafers, liquid sustenance of all forms, good wishes, media interest, bike repairers, phone fixers, encouraging messages on social media, witnesses who signed verification sheets, sponsorship, and - of course - the donations.
"The funds for Macmillan Cancer Support will help so many people who need it, and that's what matters. Thank you all."
Mrs Paterson said she took up the challenge as the "unbearable grief" she felt when sitting at home with nothing to focus on was too much.
Her son Sandy died of a heart attack in 2012, daughter Katie after suffering viral pneumonia in 2013 and son Bob in an accident in 2016. All were in their 40s.
She began backing Macmillan Cancer Support after her mother and sister died from the illness.
Last year she completed a 24-hour cycle and 10 years ago she cycled across Canada to raise money for the same charity.
Mrs Paterson said during the latest challenge: "I needed this and it's been amazing. The support we have received has been overwhelming, I never expected it."
Donations can be made at www.justgiving.com/fundraising/mavis-paterson1