This paper explores time‐modulated metasurface‐assisted waveguide setups for the unique and accurate permittivity and permeability extraction of lossy dispersive samples with phase‐less measurements. It is theoretically demonstrated that when the position of the short‐circuit termination is dynamically modulated in a predefined way, the phase information of the reflection S‐parameter manifests itself into the amplitude level of the emerging harmonics. Being insensitive to the calibration plane shifts and phase uncertainties in reflection measurements while bypassing a priori knowledge about the material under test (MUT) and also the transmission information, can be regarded as the main advantages of the proposed time‐modulated material characterization approach. Using a simple post‐processing step, several illustrative examples are presented to numerically verify the validity of the proposed approach for some applicable and practical types of homogeneous materials. Two possible realization schemes based on mechanical actuation and electrical phase control are comprehensively discussed. An uncertainty analysis to examine how realization tolerances can affect the accuracy of results is also performed. The effectiveness of the proposed method is further experimentally demonstrated using a programmable phase‐agile metasurface. By involving the temporal dimension, the proposed strategy takes a great step forward in the phase‐less reconstruction of electromagnetic parameters.