It is anticipated that after the design life of roads, the pavement materials should have extensively deteriorated necessitating reconstruction/rehabilitation. The extent to which these materials deteriorate in terms of their geotechnical properties has not been fully understood. This study evaluated the extent of deterioration of a typical marginal, mechanically stabilised lateritic crushed rock base in the Kumasi city roads after over 25 years’ in-service. As-constructed materials properties were compared with post-design life properties to determine the degree of alteration of geotechnical, chemical, and mineralogical properties. Results of the study indicate that: the in-situ mean moisture content reduced by about 16% over the service period. The grading changed significantly producing mean fines contents of over 200%. The mean liquid limit and plasticity index were 40% and 22%; equivalent to about 16% and 70% increase respectively. The in-situ-resilience moduli were between 574 and 603 MPa, indicating a significant reduction of about 81–82%. The base materials had similar chemical composition with dominant oxides being silica, alumina and iron oxides with mean concentrations of 52.4%, 28% and 6.5%, respectively. Quartz, kaolinite, albite, and rutile were the minerals identified with no swelling clays present. In conclusion, although the mechanically stabilised lateritic base material during construction did not meet the specification strictly, the pavement performed beyond the design life. In addition, despite the deterioration in material properties, the base layer generally still possesses adequate resilience moduli and mineralogical stability to enable continual use during rehabilitation, except for zones with resilience modulus less than 200 MPa, which require strengthening.