Conventional methods to extract an effective mobility ( $\mu _{\mathrm {eff}}$ ) in a metal–oxide–semiconductor field-effect-transistor (MOSFET) tend to ignore the portions of parasitic components of the device. This can cause a substantial error in the extracted value of the effective mobility. In this letter, we have proposed a unified procedure that enables to accurately capture each portion of parasitic series resistance ( $R_{{\mathrm {SD}}}$ ) and parasitic gate capacitance ( $C_{g\_{}{\mathrm {par}}}$ ) components. Then, we have investigated the impact of the parasitic components on the extracted value of the effective mobility in a gate-last surface-channel In0.7Ga0.3As quantum-well MOSFET. We have found that the extracted effective mobility using our method turns out to be independent upon gate length ( $L_{{\mathrm {g}}}$ ) from 10 to $4~\mu \text{m}$ , which verifies the accuracy of the approach proposed in this letter.