A 30-GHz integrated subharmonic mixer based on a single graphene field-effect transistor (G-FET) has been designed, fabricated, and characterized. The mixer is realized in microstrip technology on a 250- μm-high-resistivity silicon substrate. In order to enhance the current on-off ratio, the G-FET utilizes a channel consisting of an array of bow-tie structured graphene, yielding a current on-off ratio of 7. A conversion loss (CL) of 19 ± 1 dB over the frequency range of 24-31 GHz is obtained with a local oscillator (LO) to RF isolation better than 20 dB at an LO power of 10 dBm. The overall minimum CL is 18 dB at 27 GHz. The mixer has a 3 GHz ± 1-dB IF bandwidth, which is achieved with a fixed LO signal of 15 GHz. The mixer linearity is characterized and the highest third-order intercept point is measured to be 12.8 dBm.