Abstract Motivated by the stellar fossil record of Local Group (LG) dwarf galaxies, we show that the star-forming ancestors of the faintest ultra-faint dwarf galaxies (UFDs; MV ∼ −2 or M ⋆ ∼ 102 at z = 0) had ultraviolet (UV) luminosities of M UV ∼ −3 to −6 during reionization (z ∼ 6–10). The existence of such faint galaxies has substantial implications for early epochs of galaxy formation and reionization. If the faint-end slopes of the UV luminosity functions (UVLFs) during reionization are steep (α ≲ −2) to M UV ∼ −3, then (i) the ancestors of UFDs produced >50 per cent of UV flux from galaxies; (ii) galaxies can maintain reionization with escape fractions that are more than two times lower than currently adopted values; (iii) direct Hubble Space Telescope and James Webb Space Telescope observations may detect only ∼10–50 per cent of the UV light from galaxies; and (iv) the cosmic star formation history increases by ≳ 4–6 at z ≳ 6. Significant flux from UFDs, and resultant tensions with LG dwarf galaxy counts, is reduced if the high-redshift UVLF turns over. Independent of the UVLF shape, the existence of a large population of UFDs requires a non-zero luminosity function to M UV ∼ −3 during reionization.