Shawn was leaving the Jack's Wife Freda bistro in NYC Thursday with his new girlfriend, Camila Cabello, when we wished him a happy bday and asked him how he was planning to celebrate.

Mendes didn't dish out any party plans ... instead, he calmly revealed to our camera guy that simply letting him stroll down the sidewalk whilst holding hands with his dame would be a birthday wish come true.

Our photog complied ... mostly. C'mon, he had to try to get SOMETHING.

As we've reported ... Shawn and Camila have been getting hot and heavy over the past month, including a steamy makeout session in Miami Beach just over a week ago.

Before that, they were spotted smooching and snuggling on the West Coast.