There is a growing concern about the mental health of children and young people (CYP) in the UK, with increasing demand for counselling services, admissions for self-harm and referrals to mental health services. We investigated whether there have been similar recent trends in selected mental health outcomes among CYP in national health surveys from England, Scotland and Wales. Data were analysed from 140 830 participants (4-24 years, stratified into 4-12, 13-15, 16-24 years) in 36 national surveys in England, Scotland and Wales, 1995-2014. Regression models were used to examine time trends in seven parent/self-reported variables: general health, any long-standing health condition, long-standing mental health condition; Warwick-Edinburgh Mental Wellbeing Score (WEMWBS), above-threshold Strengths and Difficulties Questionnaire Total (SDQT) score, SDQ Emotion (SDQE) score, General Health Questionnaire (GHQ) score. Across all participants aged 4-24, long-standing mental health conditions increased in England (0.8-4.8% over 19 years), Scotland (2.3-6.0%, 11 years) and Wales (2.6-4.1%, 7 years) (all p < 0.001). Among young children (4-12 years), the proportion reporting high SDQT and SDQE scores decreased significantly among both boys and girls in England [SDQE: odds ratio (OR) 0.97 (0.96-0.98), p < 0.001] and girls in Scotland [SDQE: OR 0.96 (0.93-0.99), p = 0.005]. The proportion with high SDQE scores (13-15 years) decreased in England [OR 0.98 (0.96-0.99), p = 0.006] but increased in Wales [OR 1.07 (1.03-1.10), p < 0.001]. The proportion with high GHQ scores decreased among English women (16-24 years) [OR 0.98 (0.98-0.99), p = 0.002]. Despite a striking increase in the reported prevalence of long-standing mental health conditions among UK CYP, there was relatively little change in questionnaire scores reflecting psychological distress and emotional well-being.