The future King met clients, advisors and staff at the south London charity which works with offenders and disadvantaged people to help them rebuild their lives.
Around 40 per cent of the staff at St Giles have a criminal conviction and they aim to provide tailored, peer-led support to which their clients can relate to.
The new dad then had an informal lunch with chief executives from businesses within the corporate sector to discuss their support and involvement with the charity.
''This charity truly inspires me. It has touched the lives of a quarter of a million people over the past five decades," the Duke of Cambridge said at a dinner in aid of the charity last year.
"These are people from the margins of society who, thanks to this charity, were able to recover from an appalling start and go on to live successful, productive and positive lives.
"What started as a small soup kitchen for the homeless and destitute has become one of the leading charities in this country helping ex-offenders to reform, resettle, and – critically- to break out of the costly and destructive cycle of reoffending."
The 31-year-old last visited the head office in 2009. He was a Patron of their 50th anniversary year in 2012.
When William confirmed he was leaving the RAF shortly after the birth of Prince George, he also announced an increased commitment to his existing charity work.
The Kensington Palace statement said the Prince plans to "expand his work in the field of conservation particularly in respect of endangered species" and will "continue to expand his charities on issues relating to children and young people, veterans and serving members of the Armed Forces."
He will "continue to support the work of The Queen and the Royal Family through a programme of official engagements, both at home and overseas, with The Duchess of Cambridge," and over the next 12 months, will "work closely with the Royal Foundation of The Duke and Duchess of Cambridge and Prince Harry."