The future king wore the magazine's official red baseball cap and jacket as he tried to tempt passers-by in the Westminster area into buying a copy.
Richard Hannant, 47, posed for a picture with the prince that his brother-in-law, Matthew Gardner, shared on LinkedIn.
"What an honour to have a private moment with our future king who was humble and working quietly in the background, helping the most needy," wrote Gardner.
"The finale to this unique occasion was when Prince William asked my brother-in-law if he wanted to buy the Big Issue, to which he replied: 'I have no change'.
"At this point William produced a mobile card machine. You cannot teach that!"
Cab driver Neil Kramer also shared a photo with the prince selling the magazine.
The outing comes after Britain on Sunday concluded four days of celebrations honouring his grandmother Queen Elizabeth II's record-breaking 70 years on the throne.
William has long worked on homelessness issues and has been a patron of the Centrepoint charity, which works on the issue, since 2005.
In December 2009, he joined Centrepoint's CEO spending a night sleeping on the street to deepen his understanding of the challenges young homeless people face, the charity said.
The 39-year-old is the most popular royal behind the queen, with 66 percent favourability, according to YouGov, with the public warming to his low-key manner and charity work.
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