News of the hire was first reported on Tuesday by the Wall Street Journal, which also spoke to the prince.
The website for the company, which was founded in 2013 and provides mobile-based professional coaching, counselling and mentorship, lists the prince as a member of its leadership team.
The prince has been living in California with his wife Meghan, duchess of Sussex, and their son Archie for the past year.
He told the Wall Street Journal that he has taken the job so he can "help create impact in people's lives".
"Proactive coaching provides endless possibilities for personal development, increased awareness and an all-round better life," he said.
Since the pair moved to the US, they have begun forging careers outside of royal life.
The couple have set up a production company called Archewell Productions and started a podcast under the name Archewell Audio, inking deals with Netflix and Spotify.
The companies are in addition to their charitable foundation, Archewell, which aims to "uplift and unite communities".
Prince Harry and Meghan both announced they were "stepping back" as senior members of the royal family in February 2020.
In a highly-anticipated interview with Oprah Winfrey earlier in March, the pair said their decision to step back from their roles was influenced by negative and racist press coverage they faced.