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Motivated by the stellar fossil record of Local Group (LG) dwarf galaxies, weshow that the star-forming ancestors of the faintest ultra-faint dwarf galaxies(UFDs; ${\rm M}_{\rm V}$ $\sim -2$ or ${\rm M}_{\star}$ $\sim 10^{2}$ at $z=0$)had ultra-violet (UV) luminosities of ${\rm M}_{\rm UV}$ $\sim -3$ to $-6$during reionization ($z\sim6-10$). The existence of such faint galaxies hassubstantial implications for early epochs of galaxy formation and reionization.If the faint-end slopes of the UV luminosity functions (UVLFs) duringreionization are steep ($\alpha\lesssim-2$) to ${\rm M}_{\rm UV}$ $\sim -3$,then: (i) the ancestors of UFDs produced $>50$% of UV flux from galaxies; (ii)galaxies can maintain reionization with escape fractions that are $>$2 timeslower than currently-adopted values; (iii) direct HST and JWST observations maydetect only $\sim10-50$% of the UV light from galaxies; (iv) the cosmic starformation history increases by $\gtrsim4-6$ at $z\gtrsim6$. Significant fluxfrom UFDs, and resultant tensions with LG dwarf galaxy counts, are reduced ifthe high-redshift UVLF turns over. Independent of the UVLF shape, the existenceof a large population of UFDs requires a non-zero luminosity function to ${\rmM}_{\rm UV}$ $\sim -3$ during reionization.

Local Group Ultra-Faint Dwarf Galaxies in the Reionization Era