To assess the overlap and admission or transfer rate of children with chronic complex conditions (CCC) and with mental or behavioral health (MBH) disorders among children presenting to the emergency department (ED). We performed a cross-sectional analysis from 2 data sources: hospitals in the Pediatric Health Information System (PHIS) and from a statewide sample (Illinois COMPdata). We included ED encounters 2 to 21 years and compared differences in admission and/or transfer between subgroups. Among patients with both a CCC and MBH, we evaluated if a primary MBH diagnosis was associated with admission or transfer. There were 11 880 930 encounters in the PHIS dataset; 0.7% had an MBH and CCC, 2.2% had an MBH, and 8.0% had a CCC. Patients with an MBH and CCC had a greater need for admission or transfer (86.5%) compared with patients with an MBH alone (57.7%) or CCC alone (52.0%). Among 5 362 701 patients in the COMPdata set, 0.2% had an MBH and CCC, 2.1% had an MBH, and 3.2% had a CCC, with similar admission or transfer needs between groups (61.8% admission or transfer with CCC and MBH; 42.8% MBH alone, and 27.3% with CCC alone). Within both datasets, patients with both a MBH and CCC had a higher odds of admission or transfer when their primary diagnosis was an MBH disorder. While accounting for a small proportion of ED patients, CCC with concomitant MBH have a higher need for admission or transfer relative to other patients.