William has become the figurehead of The Passage, an organisation which has helped more than 130,000 people in crisis over almost 40 years through its resource centre, homelessness prevention projects and innovative accommodation services.
Diana took William and younger brother Harry to the charity in December 1993, and as a young man William has made numerous public and private trips to the organisation.
Cardinal Vincent Nichols, head of the Catholic Church in England and Wales and patron of The Passage, welcomed the announcement ahead of the duke's visit to the charity's St Vincent's Centre in Westminster.
He said: "I know the duke is deeply committed to working with those most in need, such as the thousands of people The Passage has helped off the streets.
"It is crucial that everyone in our society, especially the poorest, are treated with respect, dignity and kindness, and are given the opportunities to fulfil their potential in life; this announcement is a tangible symbol of that commitment."
William last visited the St Vincent's Centre with wife Kate in the run-up to Christmas, when he spoke out against the drug spice after hearing about its devastating effect on the capital's homeless.
The duke told Mick Clarke, chief executive of The Passage: "The fact it was a legal high and that young people were allowed to take it and it leads to addiction is just terrible."
The synthetic drug is seen as the biggest challenge the charity faces as it is cheap, readily available and can exacerbate existing problems like poor mental health.
Following the announcement Mr Clarke said: "During His Royal Highness's visits to The Passage, it has been very clear that he has a deep concern for those affected by homelessness and a real interest in our work.
"For His Royal Highness to further express his support for The Passage's work by becoming our royal patron is a tremendous honour, and a testament to how much he genuinely cares about this issue."
The duke marked becoming royal patron of a homeless charity by serving lunch to its clients - and quipped he was on trial for a job.
Wearing an apron, William dished out spaghetti bolognese to men and women who had patiently queued for the free lunch made by staff and volunteers.
Mick Clarke, chief executive of The Passage, joined William as he ladled the meat sauce on to pasta and joked to a client: "We're giving him a trial."
Stay up to date with the latest news stories from a Christian perspective. Sign up to our daily newsletter and receive more stories like this straight to your inbox every morning.
