Samantha Power was in Cameroon to show US support for the campaign against militant Islamist group Boko Haram when the accident happened on Monday.
She said she met the boy’s family to offer “profound condolences”.
An armoured jeep knocked the boy as he tried to cross a road when the convoy was heading towards a refugee camp.
“Although the boy received immediate medical care from an ambulance in our convoy, he died shortly thereafter,” Ms Power said.
US officials were unable to say whether their government will pay compensation to the boy’s family who lived in a village in northern Cameroon. AFP news agency reports.
Ms Power’s motorcade was moving at a fast speed when the sixth vehicle in it hit the boy after he darted on to the two-lane highway near the small city of Mokolo, the Associated Press news agency reports.
The vehicle that struck the boy stopped, but was then ordered by US security forces to continue travelling through the unsecured area, AP reports.
Ms Power later met children at a camp for people who fled attacks by Boko Haram, which is fighting to establish an Islamic state in the region.
“All of you who are attempting to fight this terror, the United States stands with you,” she said, AP reports.
Boko Haram is the most dangerous militant group in the region.
It launched its insurgency in northern Nigeria in 2009, but has increasingly targeted neighbouring Cameroon, Chad and Niger.
The conflict has led to the deaths of some 17,000 people, destroyed some 1,000 schools and displaced 2.5 million people.