The TV presenter leapt from Hyde Park's Wellington Arch, in central London, for Uk4u Thanks!, which is sending Christmas boxes to servicemen and women away from home this year.
Royal Marines led the charge down the zip wire, touching down to receive a Christmas box from Santa Claus.
Kelly, dressed in a Royal Marine uniform, followed screaming as she careened to the ground with three Marines ready to catch her at the bottom.
"These guys are amazing," she said while having her harness removed.
"I think today is really, really important because it's a chance for us to say 'thank you' to all of our troops."
Kelly said she had become an honorary Royal Marine today and these events show the public are really thinking of our troops overseas.
"I was screaming like a girl; shameful, wearing a Royal Marine uniform and screaming like a girl. Sheer fear, sheer terror. When I got up there I said to the Marines: 'this isn't a good time to tell you I have a fear of heights'."
Lance Sergeant Matthew Edmundson said: "I think she did all right. However, I wasn't sure about the screaming on the way down."
This year's Christmas box has been decorated in the colours of the Diamond Jubilee Medal ribbon in honour of the Queen.
Last year Uk4u Thanks! sent more than 20,000 Christmas boxes to servicemen and women around the world.
The charity allows the public to show their support to troops without putting pressure on the forces mail system.