The objective of the study was to determine the proportion of preschool children under 5 years old meeting the 24-h World Health Organisation guidelines on physical activity, sedentary behaviour and sleep. Another objective was to examine the association between the health-related quality of life and meeting these guidelines among preschool children in Singapore. Parents completed an online and anonymous SMALLQ® (Surveillance of digital Media hAbits in earLy chiLdhood Questionnaire) on children's physical activity, sedentary behaviour and sleep. Meeting the 24-h WHO guidelines meant, within a 24-h period, (i) having at least 180 min of physical activity (ii) engaging in less than 60 min of screen media and (iii) having 10-13 h of good quality sleep. Parent-reported health-related quality of life of the children was determined using the Pediatric Quality of Life Inventory™, collected online at the same time. Parent-reported data showed that 12.6% "met none" while 9.6% of preschool children met all the WHO guidelines. 70.7%, 56.9% and 26.5%, of preschool children respectively, achieved the sleep, physical activity and screen media use guidelines within a 24-h period. 40.5% met two guidelines while 37.4% "met one" guideline. Significant differences were detected in the health-related quality of life among preschool children who "met all," "none," or "met" 1-2 of the WHO guidelines (i.e. total health score: 82.9 ± 12.4 vs. 76.4 ± 15.1 vs. 78.6 ± 14.5%, p < 0.05; n[superscript 2] = 0.008-0.11). Our results show that the health-related quality of life of preschool children increased with the number of WHO guidelines accomplished.