Missing the annual event "was never an option as far as I was concerned," he told BBC News. "Everyone that was planning on coming is still here. No one's changed any plans."
Still, the memory of tragedy lingered in a hush as competitors observed a 30-second moment of silence in honor of those who didn't finish their race, reports CNN.
"The way Boston has dealt with it has been absolutely remarkable," added the prince.
Harry, 28, however, wasn't joined by Prince William or the expectant duchess, who had other volunteer obligations.
Chatting up marathon volunteers prior to handing out medals at the finish line, Harry quipped, "She's pregnant and he's getting old. It's impossible."
Kate Looks 'Blooming'Meanwhile, the Duchess of Cambridge earned some smiles of her own as she met with a group of the most accomplished scouts in the U.K.
Shining in a Mulberry mint green coat at the Windsor Castle, the volunteer scout leader, 31, handed out awards and inspected 400 youngsters who had earned status as prestigious Queen's Scouts.
"She was looking blooming, almost tanned in the sunshine," says an observer at the event.
After hearing from one boy, Lee, 17, who had saved a friend from the freezing waters of a canal, she told him she was "amazed at what you have done. Keep it up."
In her nude heels, no less, she joined the chief scout, adventurer Bear Grylls, for a procession down the cobbled paths of the castle's grounds to St. George's Chapel for the annual church service.
The source adds: "She had a real spring in her step."