This paper presents the design of a low noise Schottky-diode 190–240 GHz subharmonic mixer (SHM). The embedding impedance condition for the diodes to yield optimum circuit performance is primarily determined. It is achieved using the computer-aided design (CAD) load-pull techniques by taking into account the high-frequency parasitic effects caused by the diode chip geometry. By knowing this condition, the design procedure of the mixer circuitry is expedited. The design effectiveness of the proposed methodology is experimentally validated by the good agreement between the design predictions and the measured results of the mixer performance. Y-factor measurements show that the mixer exhibits the double side band (DSB) noise temperature lower than 1500 K and DSB conversion loss less than 10 dB over the 190–240 GHz frequency range, with the DSB noise temperature and conversion loss at 220 GHz of 800 K and 7 dB respectively.