The prince's face could be seen adorning shop windows across the world, with major chains offering big discounts after Tuesday's release.
Barnes & Noble, the world's largest bookseller, showed on Tuesday that Spare had beaten heavyweight titles such as The Stolen Heir by Holly Black, former US first lady Michelle Obama's autobiography The Light We Carry and 2022 book of the year Lessons in Chemistry to take the top spot on the retailer's Top 100 list in the US.
It also topped Amazon's bestseller list in the US and UK.
At Politics and Prose, an independent bookstore in Washington, Tuesday's first sale of the day was a copy of Spare and staff received several phone calls from people asking if it was available.
Manager Keith Vient said the shop had ordered about 200 copies of the memoir and had since seen interest spike after Prince Harry’s recent media interviews.
“It’s created a lot of buzz and hype for it,” Mr Vient told The National. “But he has done a lot of media. At what point are people going to get burnt out?”
Amazon was offering savings for US and UK customers. The book has a US cover price of $36 but was being sold at $22.42.
Leaked excerpts from the book's accidental release in Spain provided insights into Harry's duty in Afghanistan, his past cocaine use, his wife Meghan's fraught relationship with the Princess of Wales as well as his own troubled relationship with his brother, Prince William.
Spare was also performing well online in the UK, where it was among other bestsellers for Waterstones, the country's largest book seller.
It had also reached the top spots in Amazon and Barnes & Noble's best-selling books in Australia.
The Duke sat down for a torrent of interviews in promotion of the book's highly anticipated launch, teasing revelations about his relationships with his family in interviews with ITV and CNN.
But despite the online frenzy to buy the memoir, copies of the book were seen piling up in London bookshops, with price cuts already announced in some retailers, including Amazon UK, which was selling it at half-price. In Tesco, the UK’s supermarket chain, it was on sale for as little as £8.
Foyles, the largest book store in the UK by square footage, was selling the best-seller for 50 per cent off its original price of £28.
At a Barnes & Noble shop in New York's Union Station, an assistant manager said sales had been “non-stop”. The bookstore and its UK affiliate Waterstones were both offering discounts.
“I had 30 people waiting outside and all 30 went straight to the book pretty much as soon as we opened the store today,” he said.
Kat, a customer inside the store, said: "I am buying the book because all stories in the press are biased so I thought what better to find out what the real story is … I decided to buy the book."
Random House said it had printed 25 million copies of the tell-all in North America alone in response to the furore that Prince Harry's memoir has caused since its 2021 announcement.
But the Duke of Sussex must wait until next week to see if he can add the accolade “New York Times bestselling author” to his CV.
- With Adla Massoud in New York and Kyle Fitzgerald in Virginia.