  We present tomographic measurements of structure growth using
cross-correlations of Atacama Cosmology Telescope (ACT) DR6 and Planck CMB
lensing maps with the unWISE Blue and Green galaxy samples, which span the
redshift ranges $0.2 \lesssim z \lesssim 1.1$ and $0.3 \lesssim z \lesssim
1.8$, respectively. We improve on prior unWISE cross-correlations not just by
making use of the new, high-precision ACT DR6 lensing maps, but also by
including additional spectroscopic data for redshift calibration and by
analysing our measurements with a more flexible theoretical model. An extensive
suite of systematic and null tests within a blind analysis framework ensures
that our results are robust. We determine the amplitude of matter fluctuations
at low redshifts ($z\simeq 0.2-1.6$), finding $S_8 \equiv \sigma_8 (\Omega_m /
0.3)^{0.5} = 0.813 \pm 0.021$ using the ACT cross-correlation alone and $S_8 =
0.810 \pm 0.015$ with a combination of Planck and ACT cross-correlations; these
measurements are fully consistent with the predictions from primary CMB
measurements assuming standard structure growth. The addition of Baryon
Acoustic Oscillation data breaks the degeneracy between $\sigma_8$ and
$\Omega_m$, allowing us to measure $\sigma_8 = 0.813 \pm 0.020$ from the
cross-correlation of unWISE with ACT and $\sigma_8 = 0.813\pm 0.015$ from the
combination of cross-correlations with ACT and Planck. These results also agree
with the expectations from primary CMB extrapolations in $\Lambda$CDM
cosmology; the consistency of $\sigma_8$ derived from our two redshift samples
at $z \sim 0.6$ and $1.1$ provides a further check of our cosmological model.
Our results suggest that structure formation on linear scales is well described
by $\Lambda$CDM even down to low redshifts $z\lesssim 1$.
