Rydberg atomic receivers offer unique advantages for quantum sensing and microwave communication. For their deployment in high-speed communication systems, a predictive model is required. However, as the symbol rate increases, their response can no longer be captured by existing steady-state and memoryless dynamic models. Here we develop a non-Markovian memory-kernel model for a Rydberg heterodyne receiver by deriving a projected state-space description from the Nakajima-Zwanzig formalism and mapping it to a discrete symbol-level receiver channel. Using a room-temperature 133Cs vapor-cell receiver, we experimentally measure symbol statistics and calibrate the corresponding memory kernel. Across Ts = 2.5–10 µs, the model reconstructs the measured symbol statistics with an average error of 6.58%±0.74%. In the short-symbol regime, memory-kernel compensation reduces the bit-error-rate from 16.9% ± 0.5% to 3.2% ± 0.28%. This work establishes a physically grounded framework for modeling high-speed Rydberg communication receivers and provides a route toward performance optimization in quantum measurement-based receivers.