Peter Barclay, 53, was competing in an equestrian game in Williamstown, Kentucky, when the freak accident happened.
His metal tipped lance hit the ground and flipped, stabbing him in the chest, according to his brother John.
The victim, from Virginia, was reportedly trying to spear a plate on the ground at the time.
John Barclay said his sibling, who performed under the name Master Terafan Greydragon, died as he was being flown to hospital.
He added: “He died doing what he loved, but will still be missed. RIP Peter aka: Terafan Greydragon.”
Mr Barclay said his brother was a regional equestrian master for the society for creative anachronism (SCA) and next year would have risen in the organisation to the title of worldwide equestrian grand marshal.
The SCA, which is dedicated to researching and recreating the arts and skills of pre-17th-century Europe, said: “The society’s equestrian marshal, Master Terafan Greydragon, died while competing in an equestrian game at an SCA event in Kentucky.
“We have reached out to Terafan’s family to express our support for them at this moment of loss. We ask that Terafan and his family be held in your hearts.”
The SCA said it would be carrying out an investigation to try to prevent a similar accident happening in the future.
John Barclay said his brother, who was involved in the SCA for about 30 years, joined the US army after college and was stationed in England for four years.
He then returned to the US before being sent to a base in Germany for four years.
“He did medieval trips throughout Europe,” his brother said.
Peter later went back to America before retiring as a lieutenant colonel and then working as a contractor for the US army.
Mr Barclay is survived by his wife Deborah, daughters Amy and Taryth, brothers John and Charles, and father Jack.