Single and double ionisation cross-sections sigma (0n) of mercury (Hg-Hgn+, n = 1, 2) have been measured for electron-impact energies ranging from 100 to 1000 eV by time-of-flight ion analysis. The present values are compared to available data from the literature. The contribution of the two-step mechanism, in second order by the interaction between the projectile and the target for double ionisation, is analysed under the independent particle model. The region of validity of the two-step hypothesis on Hg is compared to the same reaction on helium.