Intumescent flame-retardant coatings, consisting of montmorillonite (MMT), polyethyleneimine (PEI), and ammonium dihydrogen phosphate (ADP), were prepared via one-step coassembly and applied onto cotton fabrics to improve flame retardancy. The effect of coating pH on the thermal stability, heat release behavior, and flame retardancy of the coated fabrics was investigated. The X-ray diffraction (XRD) characterization results showed that MMT could interact with PEI and ADP, forming orientated clay layers intercalated by PEI and ADP. The vertical burning test (VBT) and microscale combustion calorimeter (MCC) characterization results demonstrate that the coatings at pH7 or pH4 possessed superior flame retardant performance to the one at pH10 due to a higher remaining concentration of ADP. Additionally, the sample coated at pH7 slightly outperformed the one coated at pH4 mainly owing to a higher coating viscosity at an acidic condition. Overall, this coassembled intumescent coating can be effectively applied to improve the flame retardancy of cotton fabrics and potentially other polymers, but care must be exercised to adjust the pH value to achieve the best possible performance.