Mental health hospitalization rates among U.S. children have been increasing locally and nationally in recent decades. Children in New York State (NYS) have also witnessed several collective traumatic events during the last two decades including the September 11, 2001, terrorist attacks (9/11), the Great Recession, and Hurricane Sandy (2012) and its aftermath. Decomposition of these rates into age, period, and cohort effects may help elucidate how large-scale collective traumatic events may be driving time trends. This study examined age–period–cohort effects in children and youth mental health hospitalizations in NYS from 1999–2013. Age effects followed a linear trend from age 5 years, B = −2.76, 95% CI [−3.48, −2.03)] up to age 15 years, B = 1.62, 95% CI [1.52, 1.73]. The largest period effects were noted in 2004, B = 0.36, 95% CI [0.28, 0.45], and in 2013, B = 0.31, 95% CI [0.15, 0.47], approximately 3 years after 9/11 and the Great Recession, respectively. The largest birth cohort effect was noted for children born in 1992–1995 (range: 0.29 for children born in 1992–0.27 for children born in 1995), suggesting that the birth cohorts who experienced the 9/11 attacks during middle childhood and the Great Recession during puberty are at increased risk of mental health hospitalizations compared to other birth cohorts.