  We measure the cross-correlation of cosmic microwave background lensing
convergence maps derived from Atacama Cosmology Telescope data with galaxy
lensing convergence maps as measured by the Canada-France-Hawaii Telescope
Stripe 82 Survey. The CMB-galaxy lensing cross power spectrum is measured for
the first time with a significance of 4.2{\sigma}, which corresponds to a 12%
constraint on the amplitude of density fluctuations at redshifts ~ 0.9. With
upcoming improved lensing data, this novel type of measurement will become a
powerful cosmological probe, providing a precise measurement of the mass
distribution at intermediate redshifts and serving as a calibrator for
systematic biases in weak lensing measurements.
