The development of colistin resistance in carbapenem-resistant strains poses a serious public health problem. In this study, we collected 249 carbapenem-resistant Escherichia coli isolates from patients in Seoul in 2018, and screened all isolates for colistin resistance and for the presence of mobile colistin resistance (mcr) genes. Colistin-resistant strains were further analyzed using multilocus sequence typing, antimicrobial susceptibility testing, detection of antibiotic resistance determinants, plasmid transconjugation, and whole-genome sequencing. Three of the 249 carbapenem-resistant isolates were resistant to colistin, and mcr-1 was detected in one isolate (SECR18-0888), which belonged to sequence type 156 and was resistant to all antibiotics tested except tigecycline. The mcr-1.1 gene was located on an ~62 kb self-transferable IncI2 plasmid along with the blaCTX-M-55 gene, and the blaNDM-1, blaTEM, qepA1, and rmtB genes were additionally detected in SECR18-0888. As an extensively drug-resistant E. coli strain producing MCR-1 and NDM-1 was identified in Korea for the first time, continued monitoring of colistin resistance in carbapenem-resistant Enterobacteriaceae should be reinforced.