The application of montmorillonite nano clay (m-NC) to improve the engineering behavior of fly ash needs to be explored in detail to understand its efficacy. Therefore, the present study examined the unconfined compressive strength (UCS) and compressibility behavior of untreated and lime treated fly ash using wide ranges of m-NC (0, 1, and 2%) at different curing periods up to 28 days. The UCS values of both untreated and lime treated fly ash improved with m-NC and curing periods. It is due to the plate-like structure and high specific surface area of m-NC, which enhances pozzolanic reactivity and thus facilitates the formation of hydrated reaction products namely calcium silicate hydrate (CSH), and calcium aluminate silicate hydrate (CASH). The compressibility of fly ash increases with an increase in m-NC content. Whereas, the compressibility of lime treated fly ash increases and reduces with the addition of 1 and 2% m-NC, respectively. Detailed mineralogical and microstructural analyses were performed to elucidate the experimental results.