The 28-year-old Harry flew home in January after 20 weeks of flying Apache helicopters in Afghanistan's Helmand province, The Sun reported.
The prince said he took enemy fighters "out of the game" and that his gaming skills had made him "quite useful".
Asked whether Harry had spoken "unwisely", Karzai said: "Prince Harry is a young man. Young men do make mistakes talking. All of us have gone through that period, so let's drop it there."
Karzai was in Britain for talks with Prime Minister David Cameron and Pakistani President Asif Ali Zardari on the Afghan reconciliation process.
London: Afghan President Hamid Karzai has said Prince Harry made a "mistake" in speaking out after his tour of duty in Afghanistan against the Taliban.
First Published: Tuesday, February 05, 2013, 20:48