During the past decade, optomechanical systems have facilitated increasingly sensitive techniques for measuring a mechanical oscillator’s amplitude by coupling the oscillator to an optical field. We present a technique to measure the amplitude of a center-of-mass (COM) motion of a two-dimensional ion crystal composed of ∼100 ions in a Penning trap. The technique employs a spin-dependent, optical-dipole force to couple the mechanical oscillation to the electron spins of the trapped ions, enabling a measurement of one quadrature of the COM motion through a readout of the spin state. By sensing motion at frequencies far from the COM resonance frequency, we experimentally determine the technique’s measurement imprecision. We resolve amplitudes as small as 50 pm, 40 times smaller than the COM mode zero-point fluctuations. We demonstrate sensitivity limits set by spin projection noise and spin decoherence due to off-resonant light scattering. When performed on resonance with the COM mode frequency, the technique demonstrated here can enable the detection of extremely weak forces (<1 yN) and electric fields (<1 nV/m), providing an opportunity to probe quantum sensing limits and search for physics beyond the standard model.