Yusuf Korow, a spokesman for the county, said in a statement late Monday that two others in the ambulance had serious injuries in the incident Monday evening.
The statement said the ambulance was on its way to pick up a patient when it hit the mine, and it called the incident a "cowardly, hateful and un-Islamic act."
Garissa County recently has been the target of attacks by suspected members of the Islamic extremist group al-Shabab, which opposes Kenya's military involvement in neighboring Somalia.