We present a new method for direct measurement of magnetic fields on ultracool stars and brown dwarfs. It takes advantage of the Wing-Ford bands of FeH, which are seen throughout the M and L spectral types. These molecular features are not as blended as other optical molecular bands, are reasonably strong through most of the spectral range, and exhibit a response to magnetic fields that is easier to detect than other magnetic diagnostics, including the usual optical and near-infrared atomic spectral lines that have heretofore been employed. The FeH bands show a systematic growth as the star gets cooler. We do not find any contamination by CrH in the relevant spectral region. We are able to model cool and rapidly rotating spectra from warmer, slowly rotating spectra utilizing an interpolation scheme based on curve-of-growth analysis. We show that the FeH features can distinguish between negligible, moderate, and high magnetic fluxes on low-mass dwarfs, with a current accuracy of about 1 kG. Two different approaches to extracting the information from the spectra are developed and compared. Which one is superior depends on a number of factors. We demonstrate the validity of our new procedures by comparing the spectra of three M stars whose magnetic fluxes are already known from atomic line analysis. The low- and high-field stars are used to produce interpolated moderate-strength spectra that closely resemble the moderate-field star. The assumption of linear behavior for the magnetic effects appears to be reasonable, but until the molecular constants are better understood, the method is subject to that assumption and rather approximate. Nonetheless, it opens a new regime of very low mass objects to direct confirmation and testing of their magnetic dynamos.