The 39-year-old went without wife Meghan to Austin, Texas, as a guest of the Mercedes and Red Bull teams.
He was also seen chatting with rival Red Bull team boss Christian Horner – donning a black polo shirt and aviator sunglasses in the sweltering Austin heat.
Harry has attended a string of glitzy F1 races all around the world.
Maybe he thinks green means go.
The Duke’s wife Meghan Markle, 42, was noticeably absent from the track after the pair jetted back from a romantic holiday in Caribbean haven Canouan last week.
The source told MailOnline: “They looked happy. As Harry walked out of the shop, he slightly bumped into one of the barrels [outside] and they both giggled and Meghan reached for his hand.
“They just looked very happy to be having a holiday together.”
Archie and Lilibet were nowhere to be seen, and it appears the couple had been away for a kid-free weekend on the resort island.
The island is said to be known as where “billionaires go to escape millionaires” due to its stunning sandy beaches and upscale resorts.