Multiple studies have shown that genomic testing has a high diagnostic yield and an impact on clinical management for patients with suspected genetic conditions. Therefore, there has been a push worldwide to apply rapid genomic sequencing in critically ill neonatal and pediatric patients. The goal of this study was to investigate the practicality of applying ultrarapid genomic testing for critically ill neonatal and pediatric patients with suspected monogenic conditions in Australia. The study recruited a total of 108 patients prospectively from March 2018 to February 2019, and data were collected until May 2019. Of the 12 hospitals that acted as collaborating sites, 5 were women's hospitals, 3 women's and children's hospitals, and 4 children's hospitals. Eligible patients included those who were admitted to a neonatal or pediatric intensive care unit (NICU or PICU) and were referred to clinical genetics for a possible monogenic condition. Chromosomal microarray was a requirement before enrollment if there was a suspected chromosomal condition. Chromosomal microarrays were performed concurrently with ultrarapid exome sequencing when the probability of a positive result on microarray was low. Additionally, rapid mitochondrial genome sequencing was performed alongside ultrarapid exome sequencing if a mitochondrial condition was suspected. When possible, ultrarapid exome sequencing was performed in both parents as well as the child (trio). The primary outcome of this study measured the time from the last sample received to the ultrarapid exome sequencing report finalized. Other outcomes measured were the diagnostic yield, change in clinical management after the report was finalized, number of reports returned before hospital discharge, and time from admission to the report being finalized. Of the 108 patients enrolled, the median age was 28 days, with a range of 0 days to 17 years. Overall, 34% were female, 57% were NICU admissions, 33% were PICU admissions, and 9% were from other hospital wards. The mean time it took from the last sample received to the report being issued was 3.3 days (95% confidence interval [CI], 3.2-3.5 days). The mean time from hospital admission to the sequencing report being finalized was 17.5 days (95% CI, 14.6-21.1 days). The mean time from hospital admission to clinical genetics referral was 8.1 days (95% CI, 6.0-10.8 days). Overall, 94% of reports were issued within the goal of 5 calendar days, and 56 molecular diagnoses were made for 55 patients (51%). Among the 62 patients in the NICU, 35 had a clear molecular diagnosis from ultrarapid exome sequencing (56%), whereas in the 36 PICU patients, 17 had a clear molecular diagnosis (47%), and in the 10 patients from other hospital wards, 3 had a clear molecular diagnosis (30%). After the ultrarapid exome sequencing report was finalized, clinical management changed for 48 of the 108 patients enrolled (44%). Overall, the ultrarapid sequencing reports that revealed a clear molecular diagnosis were regarded as “very useful” or “useful” by referring providers in 52 of 55 cases (95%) and “neutral” in 3 of 55 cases (5%). Negative reports were regarded as “very useful” or “useful” in 31 of 53 cases (58%), “neutral” in 20 of 53 cases, and “not useful” in 2 of 53 cases (4%). Altogether, this study demonstrates the utility of ultrarapid exome sequencing in critically ill pediatric patients with a suspected monogenic condition in Australia.