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We present a high-significance cross-correlation of CMB lensing maps from the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) with spectroscopically calibrated luminous red galaxies (LRGs) from the Dark Energy Spectroscopic Instrument (DESI). We detect this cross-correlation at a significance of 38; combining our measurement with the Planck Public Release 4 (PR4) lensing map, we detect the cross-correlation at 50. Fitting this jointly with the galaxy auto-correlation power spectrum to break the galaxy bias degeneracy with , we perform a tomographic analysis in four LRG redshift bins spanning to constrain the amplitude of matter density fluctuations through the parameter combination . Prior to unblinding, we confirm with extragalactic simulations that foreground biases are negligible and carry out a comprehensive suite of null and consistency tests. Using a hybrid effective field theory (HEFT) model that allows scales as small as , we obtain a 3.3% constraint on from ACT data, as well as constraints on that probe structure formation over cosmic time. Our result is consistent with the early-universe extrapolation from primary CMB anisotropies measured by Planck PR4 within 1.2. Jointly fitting ACT and Planck lensing cross-correlations we obtain a 2.7% constraint of , which is consistent with the Planck early-universe extrapolation within 2.1, with the lowest redshift bin showing the largest difference in mean. The latter may motivate further CMB lensing tomography analyses at to assess the impact of potential systematics or the …