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A silicon nanophotonic Mach-Zehnder interferometer (MZI) is used to detect the mechanical resonance of a cantilever external to a nanophotonic waveguide. Small cantilever devices, below the cut-off for waveguide supported modes, are fabricated∼ 140 nm away from one MZI arm. Cantilever resonant frequencies up to 60 MHz are measured with mechanical quality factors around 20 000 and signal to noise ratios up to 1000. Phase-locked loop frequency stability measurements indicate a mass sensitivity of 2 zg in an example cantilever of 0.5 pg mass. An interferometric transduction mechanism is confirmed, and the system is shown to work effectively in all-optical operation.