Despite anxiety being a prevalent mental health problem in children, little data exist on the pervasiveness and levels of anxiety symptoms in kindergarteners. Data from the Early Development Instrument, a teacher-completed, population-level measure of child development, were collected across Canada from 2004 to 2015. The final analytic sample consisted of 974,319 children of whom 2.6% were classified as “highly anxious”. Compared to children who exhibited “few to none” anxious behaviors, highly anxious children were more likely to be male, have English/French as a second language, and have a special needs designation. Furthermore, compared with their less anxious peers, highly anxious children had between 3.5 and 6.1 higher odds of scoring below the 10th percentile cut-off in physical, social, language/cognitive and communication domains. Our findings suggest that anxious behaviors are related to children’s overall health and illustrate the consistency and extensiveness of anxiety at a very young age among Canadian children.