It has been shown that gene mutations which drive the development of malignant melanoma (MM) in humans also lead to emergence of MM when engineered mice. However, little attention has been paid to the clinical and histopathological features of melanocytic lesions and their natural history in a given mouse model. This knowledge is crucial to enable us to understand how engineered mutations influence the initiation and evolution of melanocytic lesions, and/or for the use of mice as a preclinical model to test specific treatments. We recently reported the development of melanocytic proliferations along the spectrum of naevi to MM in a Cdk4 R24C/R24C ::Tyr- NRAS Q 61K mouse model. In this study, we followed the development of lesions over time using digital photography and dermoscopy with the aim to correlate the clinical and histopathological features of lesions developing in this model. We identified two types of lesions. The first are slow-growing dermal MMs that emanate from dermal naevi. The second did not emanate from naevi, grew rapidly, and appeared to be solely confined to the subcutaneous fat. We present a simple staging system for the MMs that progress from naevi, based on depth of extension into the dermis and subcutis. This represents a blueprint for documentation and follow-up of MMs in the live animal, which is critical for the proper use of murine melanoma models.