We study the imprint of dark matter (DM) annihilation on the global 21 cm signal from the Dark Ages to Cosmic Reionization. Motivated by recent observations, we focus on three DM candidates: (i) a 10 GeV Bino-like neutralino; (ii) a 200 GeV Wino and (iii) a 1 TeV heavier particle annihilating into leptons. For each DM candidate we assume two values for the thermally averaged annihilation cross-section 〈σv〉, the standard thermal value 〈σv〉th = 3 × 10−26 cm3 s−1 and the maximum value allowed by WMAP7 data, 〈σv〉max. We include the enhancement of DM annihilations due to collapsed structures, detailed estimates of energy deposition into the intergalactic medium (IGM), as well realistic prescriptions for astrophysical sources of UV and X-ray radiation. In these models, the additional heat input from DM annihilation suppresses the mean 21 cm brightness temperature offset by δT b ∼ a few-100 mK. In particular, the very deep δT b ∼ −150 mK absorption feature at ∼20 z 25 predicted by popular models of the first galaxies is considerably reduced or totally erased by some of the considered DM candidates. Such an enhancement in IGM heating could come from either DM annihilations or a stronger-than-expected astrophysical component (i.e. abundant early X-ray sources). However, we find that the two signatures are not degenerate, since the DM heating is dominated by haloes several orders of magnitude smaller than those hosting galaxies, whose fractional abundance evolves more slowly resulting in a smaller gradient: dδT b/dν 4 mK MHz−1 in the range ν ∼ 60-80 MHz. The detection of such signals by future radio telescopes would be clear evidence of DM energy injection at high redshifts.