An elliptical microelectromechanical system (MEMS) membrane mirror is electrostatically actuated to dynamically adjust the optical beam focus and track the axial scanning of the coherence gate in a Doppler optical coherence tomography (DOCT) system at $8\mathrm{kHz}$ . The MEMS mirror is designed to maintain a constant numerical aperture of $\sim 0.13$ and a spot size of $\sim 6.7\mathrm{\mu }\mathrm{m}$ over an imaging depth of $1\mathrm{mm}$ in water, which improves imaging performance in resolving microspheres in gel samples and Doppler shift estimation precision in a flow phantom. The mirror’s small size $(1.4\mathrm{mm}\times 1\mathrm{mm})$ will allow integration with endoscopic MEMS-DOCT for in vivo applications.