A mode converter for whispering gallery mode gyrotrons has been designed and experimentally demonstrated to have 95% calorimetric efficiency. Experiments were performed on a megawatt power level, 3μs pulsed gyrotron operating in the TE16,2,1 whispering gallery mode at 146 GHz. The gyrotron cavity employs a nonlinear uptaper to minimize radial mode conversion. About 97% of the output power is in the TE16,2 mode. A doubly curved reflector, designed using ray optics and vector diffraction theory, was recently built to focus the full radiation pattern. Experiments show that the converter focuses at least 92% of the incident radiation into a Gaussian-like TE16,2 focal spot, about 1.5 cm in width. Small fractions of other spatial modes were found to form focal spots in the far field. Analysis of the power in the other focal spots allows for a good quantitative measurement of the gyrotron output mode content, potentially on a single shot basis.