We use data from Gaia's second data release (DR2) to constrain the initial-final mass relation (IFMR) for field stars with initial masses 0.9 min/M 8. Precise parallaxes have revealed unprecedented substructure in the white dwarf (WD) cooling sequence on the color-magnitude diagram (CMD). Some of this substructure stems from the diversity of WD atmospheric compositions, but the CMD remains bimodal even when only spectroscopically confirmed DA WDs are considered. We develop a generative model to predict the CMD for DA WDs as a function of the initial mass function, stellar age distribution, and a flexibly parameterized IFMR. We then fit the CMD of 1100 bright DA WDs within 100 pc, for which atmospheric composition and completeness are well understood. The resulting best-fit IFMR flattens at 3.5 min/M 5.5, producing a secondary peak in the WD mass distribution at mWD ∼ 0.8 M . Our IFMR is broadly consistent with weaker constraints obtained from binaries and star clusters in previous work but represents the clearest observational evidence obtained to date of theoretically predicted nonlinearity in the IFMR. A visibly bimodal CMD is only predicted for mixed-age stellar populations: in single-age clusters, more massive WDs reach the bottom of the cooling sequence before the first lower-mass WDs appear. This may explain why bimodal cooling sequences have thus far evaded detection in cluster CMDs.