Climate change is associated with rising temperatures and changing rainfall patterns, which can degrade soil and reduce crop productivity. In this study, we assess the effects of rainfall and temperature-based extreme climatic indices on rice and wheat crops across all 38 Bihar districts for the historical period (1998–2019). Results indicate that rice and wheat yield trends are increasing except for Arwal, where a decrease in wheat yield was observed. Increasing yield is because of improved agricultural practices like high-yielding seeds, fertilizer, tillage, and modern technologies. Trend analysis for indices shows a decreasing trend in the number of colder days during the kharif season (TX10P_KHARIF, TN10_KHARIF) and consecutive wet days (CWD) in most districts, whereas an increase in warmer days and higher daily temperature extremes, attributable to erratic rainfall and droughts. Analyzing the relationship between climate extremes and crop yields, positive correlations were observed with precipitation indices, which boost plant growth and soil microbial activities, enhancing productivity. Specifically, wheat benefited from increased precipitation. However, specific temperature indices, like summer days (SU), days when minimum temperature > 90th percentile (TN90P), and days when maximum temperature > 90th percentile (TX90P), negatively impacted yields due to elevated temperatures causing early plant maturity and affecting kernel weight. These findings offer valuable insights for decision-makers aiming to formulate effective adaptation and mitigation policies to address the impacts of climate change on crop yield in Bihar. More studies are needed to understand how these climate extremes affect plants’ phenological stages and yield.