Louis is not a traditional Windsor first name but has a deep significance for the royal family, a historian has said.
Judith Rowbotham said the choice was a "little surprising" but remained a safe option, and would delight both the Prince of Wales and the Duke of Edinburgh.
"It's not a traditional Windsor name but it's very much an acknowledgement of a wider heritage," Dr Rowbotham said.
"I can't think of a Louis in the immediate royal family with Louis as a first name."
She added: "Louis is a little surprising on one level because it's not a strong Anglo-Saxon, British name and that has been what the royal family has tended to go for.
"Either they've gone completely off piste before with something like Zara (Princess Anne's daughter) or they've stuck to more traditional names.
"But this is a name with a provenance."
Earl Mountbatten, whose first name was Louis, was the Prince of Wales's beloved great-uncle and mentor, and the Duke of Edinburgh's uncle, but he was murdered by the IRA in 1979.
Dr Rowbotham, a visiting research fellow at Plymouth University, said both Charles and Philip would be touched by the choice.
"It's very much an affectionate tribute to the Prince of Wales," the historian said.
"Louis Mountbatten is someone who Prince William himself never knew but at the same time he has remained a very important figure in terms of the impact he had on Prince Charles' life.
"Prince Philip was also deeply devoted to Lord Mountbatten.
"At one stroke, Louis pleases great-grandfather and grandfather, which is a good thing."
Louis is also one of William's middle names, and one of Prince George's.
Dr Rowbotham said. "One of the things that they are very conscious of is the need to be seen as a stable force ... Be safe and don't raise eyebrows."