The complex dielectric constants of mixed Cs0.80(NH4)0.20H2AsO4 single crystals have been measured in the temperature range from 13 to 200 K and the frequency range from 4.4 to 624 kHz along the a and c axes. Proton-glass behavior is first observed in the low-temperature region. The asymmetric concentration-temperature (x-T) phase diagram is obtained by measuring the complex dielectric constants of mixed Cs1-x(NH4)xH2AsO4 microcrystalline powder pellets for the full range of mixing concentration x from 0 to 1. Proton-glass behavior develops in the concentration range of 0.12 < x < 0.51. Coexistence of the proton-glass phase with the respective long-range ordering is observed in the two phase boundary regions.