The Duke of Sussex has said he had to step back from royal duties to protect himself and his family from the "toxic" situation created by the UK press.
Prince Harry told TV chat show host James Corden it was "destroying my mental health" and he "did what any husband [or] father would do".
Buckingham Palace confirmed last week that the prince and his wife Meghan would not resume royal duties.
The duke insisted they were "stepping back rather than stepping down".
The couple, who are expecting their second child, moved to California with their one-year-old son Archie after announcing their plan to step back as senior working royals last January.
They said at the time that they wanted to work to become financially independent.
Last week, Buckingham Palace said the duke and duchess would not return to royal duties, following a planned review of the arrangements.
As a result, they will have to return their honorary military appointments and royal patronages.
In a segment for Corden's US TV programme The Late Late Show, the duke joined the British presenter on a double-decker bus tour of Los Angeles.
Asked by Corden about the couple's decision to walk away from royal duties, Prince Harry insisted it "was never walking away, it was stepping back rather than stepping down".
"It was a really difficult environment as a lot of people saw. We all know what the British press can be like. And it was destroying my mental health. I was like, this is toxic."
He said he "did what any husband [or] father would do", and moved his family away, but insisted: "I will never walk away, I will always be contributing. My life is public service."
Asked by Corden how he sees his future after lockdown restrictions are lifted, the duke said: "My life is always going to be about public service and Meghan signed up to that."
The interview was recorded before it was confirmed Harry and Meghan would not resume royal duties.
By Sarah Campbell, BBC News royal correspondent
Much has been said and written about Harry and Meghan over the past year, but this is Harry in his own words - the first interview since the family moved to California.
There's enough fluff to delight their fans - like details about Archie's first word and his love of waffles. Meghan makes an appearance via FaceTime and Harry gets muddy on an assault course.
The tone is informal - two mates chatting on a bus (James Corden was at their wedding) but he does ask the question people want answered, about why they are no longer working royals.
Harry is perhaps a little defensive in his response, saying it "was never walking away", and although this was recorded before the official "decree absolute" from Buckingham Palace last week, the sentiment about continued public service is the same.
Commentators will point to the name check for Netflix (the couple have signed a multi-million dollar deal with the streaming service).
Then there's the fact that in-between revealing personal family details he argues the reason they left the UK was because of press intrusion.
Next weekend, it will be Meghan who takes centre stage with Oprah.
The Duke and Duchess of Sussex have both taken legal action against media outlets in the past.
The duchess recently won a High Court privacy battle against the Mail on Sunday over the publication of extracts from a letter to her father.
When his wife first began her legal action against the Mail on Sunday, the duke described the "painful" impact of the "ruthless" press campaign against her.
"I lost my mother and now I watch my wife falling victim to the same powerful forces," he said in a statement at the time.
Referring to his late mother Diana, Princess of Wales, Harry said his "deepest fear is history repeating itself".
Earlier this month, Harry also successfully sued the publisher of the Mail on Sunday for libel, over false claims he "turned his back" on the Royal Marines after stepping down as a senior royal.
In a wide-ranging interview, the duke also defended Netflix drama The Crown, amid criticism from some quarters about its portrayal of the Royal Family's history.
"Of course it's not strictly accurate, but, loosely, it gives you a rough idea about what that lifestyle, what the pressures of putting duty and service above family and everything else, what can come from that," he said.
"I am way more comfortable with The Crown than I am seeing the stories written about my family, or my wife or myself."
Last September, it emerged the duke and duchess had agreed a deal with Netflix to make a range of programmes, some of which they may appear in.
Harry also revealed that Archie's first word was "crocodile" and the Queen sent him a waffle maker for Christmas.
He added that the monarch, 94, and the Duke of Edinburgh, 99, who is currently being treated in hospital for an infection, have used Zoom to video call the family, and have seen Archie "running around".