This paper presents the design and measurement results of a novel frequency-mixing transimpedance amplifier (FM-TIA), which is the key building block towards a monolithically integrated optical sensor front-end for frequency domain near-infrared (NIR) spectroscopy (FD-NIRS). The FM-TIA employs a T-feedback network incorporating a gate-controlled transistor for resistance modulation, enabling the simultaneous down-conversion and amplification of the high frequency modulated photodiode (PD) current. The proposed FM-TIA is capable of operating either in the traditional wideband mode or the frequency-mixing mode, depending on the applied gate control voltage. A wideband post amplifier is implemented on chip to characterize both modes for comparative study. The wideband mode achieves 107 dB Omega transimpedance gain with 200 MHz bandwidth for 4 pF photodiode capacitance. The measured total integrated input referred current noise is 158 nA(rms). When the TIA is modulated by a 100 MHz signal with 0.5 V amplitude in the mixing mode, it achieves 92 dB Omega conversion gain. The measured 1 dB compression point is 3.1 mu A and IIP3 is 10.6 mu A. The input-referred current noise integrated up to 50 kHz is only 10.4, nA(rms), which is 15 times lower than the wideband mode noise. The FM-TIA together with the post amplifier draws 23 mA from a 1.8 V power supply, where the output buffer consumes 16.15 mA.