An airborne survey system named the MARK-A1 was developed to be mounted on a commercial drone for the purpose of application in contaminated areas with high dose rate levels. The MARK-A1 system consists of a CZT detector, signal processing unit, and positioning and interface units to a PC on the ground. The weight of the system is below 1 kg for loading on a commercial drone. For experimental verification, irradiation testing and field testing were conducted. From the irradiation test, we derived the G-factor and dose rate calibration factor to calculate ambient dose rates from measured energy spectra with an applicable energy range. An airborne survey was then conducted to check the system performance at a height of about 1 m and 10 m. The ambient dose rate was successfully estimated from the calculation of the measured energy spectra and dose conversion factor. Finally, the results were compared with those from in situ gamma-ray spectrometry using a tripod at the same survey site. The difference between the two systems was below 20%, indicating a good performance of the MARK-A1. It is anticipated that the MARK-A1 can be used to quickly scan wide, highly contaminated areas that are difficult for humans to access in the event of a radiological accident.