Achieving stability of drilling fluids (DFs) rheology with high temperature and high pressure (HTHP) has always seen a growing focus with increasing pursuits of deep drilling operations to maximize hydrocarbon recovery. Since there is no boundary of how deep the drilling technology can be pushed to access deep-lying reservoirs, the quest to improve and stabilize the drilling fluids rheology is an open-ended and ongoing matter. This paper presents an investigation of two distinct clays namely sepiolite (SP) and attapulgite (AT) in nanoform as water-based drilling fluid (WBDF) additives for improved and stable rheological properties. The process of material sourcing, characterization, development in nanoform, and testing as a drilling fluid additive at low and high temperature and pressure conditions are the main focus of this study. Through experiments, it was determined that 30–60 nm size range and 4 wt% concentration of developed nanomaterials yielded the optimal performance. Various tests were then performed at HTHP, and the stability of nanosepiolite (NSP) and nanoattapulgite (NAT) in 4 wt% concentration was compared with regular drilling fluid additive (bdf403) which is used as a common rheology stabilizer in the industry. It was found that for NSP and NAT-modified drilling fluids, the yield point, plastic viscosity, and gel strength were found to be retained at temperatures and pressures of up to 180 °C and 15 ksi, respectively, in contrast to bdf403 WBDF which deteriorated at much lower temperature and pressure. NSP additives were found to be more effective than NAT additives. The results confirmed a strong dependence of the DF stability on the morphological characteristics of the tested clays, implying that the properties of the DFs can be tailored by modifying the clay morphologies, particularly in the nanoform.