Background Previous evidence indicates that mental health problems are becoming more common for adolescents. Less is known about whether these trends have continued and there has been no study to date which has specifically focused on early adolescents over a sufficiently long period. This study examines changes in parent‐ and teacher‐reported mental health problems among 10‐ and 11‐year‐olds in 1999, 2004 and 2012 in Great Britain. Method Parent and teacher ratings of the Strengths and Difficulties Questionnaire were used to compare the prevalence of conduct problems, hyperactivity/inattention, emotional problems, peer problems and total difficulties among 10‐ and 11‐year‐olds in three nationally representative British samples assessed in 1999 (n = 1904), 2004 (n = 1348) and 2012 (n = 11,397). Results Teacher reports showed improving trends for boys’ and girls’ mental health from 1999 to 2012, particularly for externalizing behaviours (i.e. conduct problems and hyperactivity/inattention). Parent reports, on the other hand, identified only one area of sustained improvement between 1999 and 2012, namely hyperactivity/inattention among boys. Although parent reports of girls’ mental health indicate improving trends from 1999 to 2004, they also suggest worsening mental health from 2004. Conclusions These findings suggest that perceptions of emotional and behavioural problems vary by the gender of the adolescent, the context in which they are observed or by whom they are reported.