The inelastic mean free path λMFP is a critical parameter for electronic devices. Here, we demonstrate Kelvin probe force microscopy (KPFM) as a technique for studying λMFP in biased, semi-metallic single-walled carbon nanotubes (SWNTs). Having one of the longest room-temperature λMFP values of any known material, SWNTs provide a unique platform for probing mesoscopic transport. KPFM directly determined λMFP as a function of bias, quantitatively determined the contributions of different scattering mechanisms, and enabled comparative study of individual SWNTs with and without disorder. The room-temperature mean free paths for optical phonon and surface plasmon-polariton scattering were measured to be 62 ± 20 nm and 260 ± 50 nm, respectively. The optical phonon scattering length is significantly longer than inferred from previous measurements, and it resolves a longstanding discrepancy between SWNT theory and experiment, suggesting that KPFM could become a preferred quantitative technique for studying transport in nanoscale systems. © 2014 American Physical Society.