The mental health and well-being of children and adolescents are critical public health concerns globally. This cross-sectional study analysed nationally representative data from the combined 2016-2022 National Survey of Children's Health (n = 239,534) to produce estimates of parent-reported diagnoses of children's anxiety, depression and behavioural or conduct problems, and to examine temporal trends for each condition. Bivariate analyses and multivariable logistic regression were used to examine the prevalence of the conditions by sociodemographic characteristics. Among children aged 3-17 years, 8.4% had anxiety, 3.7% had depression and 7.1% had behavioural or conduct problems. Between 2016 and 2022, there were increases in the prevalence of anxiety (6.9%-10.6%), depression (3.1%-4.6%) and behavioural or conduct problems (7.2%-7.5%). Older age and higher poverty were associated with higher odds of each condition, while having private insurance was associated with lower odds. Females had higher odds of anxiety (AOR = 1.22; 95% CI, 1.15-1.29) and depression (AOR = 1.48; 95% CI, 1.35-1.61), and lower odds of behavioural or conduct problems (AOR = 0.45; 95% CI, 0.41-0.48). Prevention, early intervention and improved access to mental health services are critical to reversing the ongoing trends of increasing prevalence of mental health conditions among children and adolescents.