  We study how 21 cm intensity mapping can be used to measure gravitational
lensing over a wide range of redshift. This can extend weak lensing
measurements to higher redshifts than are accessible with conventional galaxy
surveys. We construct a convergence estimator taking into account the
discreteness of galaxies and calculate the expected noise level as a function
of redshift and telescope parameters. At $z \sim 2-3$ we find that a telescope
array with a collecting area $\sim 0.2 \, {\rm km}^2$ spread over a region with
diameter $\sim 2 \, {\rm km}$ would be sufficient to measure the convergence
power spectrum to high accuracy for multipoles between 10 and 1,000. We show
that these measurements can be used to constrain interacting dark energy
models.
