The Duke of Cambridge's work with Shout 85258 — an around-the-clock text messaging helpline developed by the Royal Foundation — was made public to mark Volunteers Week.
Last month, he told fellow volunteers in a video call that was shared on social media late Friday: “I’m going to share a little secret with you guys, but I’m actually on the platform volunteering."
William, who is second in line to the throne, is one of more than 2,000 volunteers who have been formally trained to help those in need.
More than 300,000 text conversations have taken place between volunteers and people needing mental health support, with around 65% of those texting aged under 25.
William's wife, Kate Middleton, the Duchess of Cambridge, has also been helping others by taking part in “check in and chat” calls with people self-isolating or vulnerable during the pandemic.
The Associated Press contributed to this report.