We present cosmological constraints from a gravitational lensing mass map covering 9400 deg 2 reconstructed from measurements of the cosmic microwave background (CMB) made by the Atacama Cosmology Telescope (ACT) from 2017 to 2021. In combination with measurements of baryon acoustic oscillations (BAO, from SDSS and 6dF), we obtain the amplitude of matter fluctuations σ 8 = 0.819 ± 0.015 at 1.8% precision, S 8 ≡ σ 8 (Ω m /0.3) 0.5 = 0.840 ± 0.028 and the Hubble constant H 0 = (68.3 ± 1.1) km s −1 Mpc −1 at 1.6% precision. A joint constraint with CMB lensing measured by the Planck satellite yields even more precise values: σ 8 = 0.812 ± 0.013, S 8 ≡ σ 8 (Ω m /0.3) 0.5 = 0.831 ± 0.023 and H 0 = (68.1 ± 1.0) km s −1 Mpc −1 . These measurements are in excellent agreement with ΛCDM-model extrapolations from the CMB anisotropies measured by Planck. To compare these ACT CMB lensing constraints to those from the KiDS, DES, and HSC galaxy surveys, we revisit those data sets with a uniform set of assumptions, and find S 8 from all three surveys are lower than that from ACT+Planck lensing by varying levels ranging from 1.7-2.1σ. These results motivate further measurements and comparison, not just between the CMB anisotropies and galaxy lensing, but also between CMB lensing probing z ∼ 0.5 − 5 on mostly-linear scales and galaxy lensing at z ∼ 0.5 on smaller scales. We combine our CMB lensing measurements with CMB anisotropy data to constrain extensions of ΛCDM, limiting the sum of the neutrino masses to m ν < 0.12 eV (95% c.l.), for example. We also provide constraints independent of the CMB anisotropy data from Planck, using instead WMAP and ACT CMB data to constrain the spatial curvature density to −0.017 < Ω k < 0.012 (95% c.l.) and the dark energy density to Ω Λ = 0.68±0.01. We describe the mass map and related data products that will enable a wide array of cross-correlation science. This map is signal-dominated for lensing mass map multipoles L < 150 with the large scales measured with a signal-to-noise ratio around twice that of Planck. Our results provide independent confirmation that the universe is spatially flat, conforms with general relativity, and is described remarkably well by the ΛCDM model, while paving a promising path for neutrino physics with gravitational lensing from upcoming ground-based CMB surveys. 5 https://github.com/carronj/planck PR4 lensing 0.750 0.775 0.800 0.825 0.850 0.875 8 ACT lensing: baseline ACT lensing: inpaint SZ ACT lensing: polarization only ACT lensing: CIB deproj. ACT lensing: linear theory Planck CMB aniso.