The present randomised clinical trial was aimed at comparing three minimally invasive restorative treatment approaches for managing dental caries in occlusal surfaces using a non-gamma-2 amalgam and a low-viscosity glass-ionomer as the restorative material. The treatment approaches tested in parallel groups were: conventional in a university setting, modified-conventional and ultraconservative (Atraumatic Restorative Treatment, ART) approaches in a field setting. A split-mouth design was used in which the two restorative materials were randomly placed in 430 matched contralateral pairs of permanent molar teeth. A total of 152 children from five primary schools were recruited and treated by a dental therapist. The restorations were evaluated after 6 years by 2 calibrated independent examiners. The 6-year successes for all occlusal amalgam and glass-ionomer restorations were 72.6 and 72.3%, respectively. There were no statistically significant differences observed between the successes for both amalgam and glass-ionomer restorations placed either by the ART (68.6%, with 95% CI = 61-76%) approach or by the conventional (74.5%, with 95% CI = 65-82%) and the modified-conventional (75.8%, with 95% CI = 67-83%) approaches after 6 years. There was also no statistically significant difference observed between the successes of occlusal ART restorations with glass-ionomer (67.1%, with 95% CI = 56-77%) and occlusal conventional restorations with amalgam (74%, with 95% CI = 61-85%) after 6 years. 'Restoration fracture/marginal defects' and 'loss of material' were the most common causes for failure. The former was more often recorded in amalgam restorations and the latter in glass-ionomer restorations. Secondary caries was observed for 2% of glass-ionomer and for 10% of amalgam restorations. This difference was statistically significant (p = 0.001). The ART approach using glass-ionomer performed equally well as conventional restorative approaches using electrically driven equipment and amalgam for treating dentinal lesions in occlusal surfaces after 6 years.