The Duke appeared at the event to present Pittsburgh Steelers defensive lineman Cam Heyward with the Walter Payton Man of the Year Award.
His presence at the ceremony comes after Harry flew thousands of miles from his home in Montecito, California to be at the King's side on Tuesday, meeting with his father for around 45 minutes at Clarence House.
Harry appeared in public for the first time since his father's diagnosis on Thursday. While introducing the award, he joked that the United States "stole rugby from us and you made it your own".
"Instead of passing it backwards, just pass it forwards," he continued. "Why not wear pads and a helmet?
"All kidding aside, what you guys do on and off the field is truly remarkable. You are role models for millions in the way you carry yourselves and the way you give back. This final award, the highest honour, is all about serving your community, and there is one special man we'd like to pay tribute to now."
Cam Heyward couldn't hide his surprise when receiving the award. After hugging the prince, he said: "Prince freakin Harry, I'm just shocked, that's Prince Harry."
Following Harry's reunion with his father, he was spotted at London's Heathrow Airport on Wednesday afternoon. Meanwhile, The Duchess of Sussex remained at the couple's home in Montecito with their children, Prince Archie, four, and Princess Lilibet, two.
Less than an hour after Harry arrived at Clarence House, the King and Queen were pictured being driven to Buckingham Palace, where a helicopter took them to their Sandringham home in Norfolk.
It's not known where Harry stayed during his UK visit, with Buckingham Palace confirming last June that the Sussexes had vacated their official UK residence, Frogmore Cottage.
Now that he's back in America, Harry will no doubt be preparing for his trip to Canada next week. The Sussexes are set to travel from 14 to 16 February for the Invictus Games Vancouver Whistler 2025's One Year to Go celebrations.
Over the course of three days in Vancouver and Whistler, HELLO! understands that Harry and Meghan will join members of the Participating Nations Winter Training Camp, which provides an opportunity for members of the International Invictus Community, including team managers, coaches and competitors to experience winter adaptive sports ahead of the Games next year.