Diarrheic calves can harbor E. coli with diverse antimicrobial resistance and virulence characteristics, complicating disease treatment and control. This study characterized antimicrobial resistance, resistance and virulence genes, phylogenetic groups, and experimental mouse lethality of E. coli isolated from diarrheic calves on two large-scale cattle farms in Hebei Province, China. Fecal samples from 102 diarrheic calves were examined. Antimicrobial susceptibility was tested by the Kirby–Bauer disk diffusion method, resistance and virulence genes and phylogenetic groups were determined by PCR, and lethality was evaluated in a mouse challenge model. Forty-five E. coli isolates were recovered (44.1%), mainly from phylogroup B1 (64.4%), followed by A (22.2%) and B2 (13.3%). Resistance was highest to ceftriaxone (26.7%), followed by ceftazidime (24.4%), ampicillin (22.2%) and tetracycline (13.3%), and 15.6% of isolates were multidrug resistant. blaTEM was detected in all isolates, based on generic PCR detection, while sul1, tetA, blaCTX-M-1, tetB, blaCTX-M-55, and qnrS occurred at 2.2–17.8%. Based on PCR detection of toxin-associated markers, isolates were assigned as molecular ETEC (n = 9), molecular STEC (n = 10), molecular ETEC/STEC hybrids (n = 6), or Other (n = 20). Twenty isolates (44.4%) showed high lethality in mice. These findings reveal substantial heterogeneity in antimicrobial resistance, virulence, and experimental lethality among calf-derived E. coli.