In this study, a combination of geostatistical methods and water quality indices were used to analyze long-term data on the water quality of Sumendu Lake, also known as Mirik Lake, a major tourist attraction in the Darjeeling Himalaya of West Bengal, India. The emphasis was on using descriptive statistics to analyze water quality, as well as indexing approaches to classify water quality and organic pollution levels of Mirik lake over an extended period. Multivariate statistical analyses were used to identify the influencing factors. According to the results of the WQI analysis, the lake’s water quality is rated as “good” to “moderate” using the weighted arithmetic water quality index technique, which ranges from 36.38 to 68.99. The calculated organic pollution index has shown a Class II to Class III level of contamination (i.e. beginning of contamination to lightly polluted) during the observation period except 2017 (OPI = 5.68, heavily polluted). The findings of the research illustrate the beginning of organic pollution and a eutrophic condition in the monitored lake water, which may result in the deterioration of water quality and the death of the lake in the near future. Therefore, these challenges must be addressed immediately, and effective preventive measures must be implemented to protect this water body and the nearby community that relies on it. Evaluation and interpretation of long-term data on the water quality of Mirik Lake may produce vital information that is critical for water management planning, enabling more comprehensible water quality projections and conservation measures to protect this water resource.