The etiquette expert then went on to criticise the new father by suggesting his look wasn't suitable for the fancy occasion.
Joining Prince Harry on the yellow carpet at Leicester Square for the UK premiere of The Lion King, was wife, Meghan Markle.
Harry's lack of formality in his dress wasn't the only thing that drew criticism from the Duke and Duchess of Sussex.
When meeting music royalty Beyoncé and husband Jay-Z at the European premiere of the Disney live-action remake, Meghan Markle opted to hug Beyoncé, a move that drew the ire of many royal commentators.
Speaking to E! News, Mr Sylvester said: "I just think she's doomed, like she's damned if she does and she's damned if she doesn't.
"If she takes a photo with Beyoncé everyone's going to say, she's so Hollywood, this is not what royals do. If she doesn't take the photo with Beyoncé, everyone is going to say she snubbed her, she's too good for her."
Meghan and Harry left their first child, baby Archie Harrison, at home to attend the star-studded event.