Cefalexin is a widely used 1st generation cephalosporin, and resistance in Escherichia coli is caused by Extended-Spectrum (e.g., CTX-M) and AmpC β-lactamase production and therefore frequently coincides with 3rd generation cephalosporin resistance. However, we have recently identified large numbers of E. coli isolates from human infections, and from cattle, where cefalexin resistance is not β-lactamase mediated. Here we show, by studying laboratory selected mutants, clinical isolates, and isolates from cattle, that OmpF porin disruption or downregulation is a major cause of cefalexin resistance in E. coli. Importantly, we identify multiple regulatory mutations that cause OmpF downregulation. In addition to mutation of ompR, already known to downregulate OmpF and OmpC porin production, we find that rseA mutation, which strongly activates the Sigma E regulon, greatly increasing DegP production, which degrades OmpF, OmpC and OmpA. Furthermore, we reveal that mutations affecting lipopolysaccharide structure, exemplified by the loss of GmhB, essential for lipopolysaccharide heptosylation, also modestly activate DegP production, resulting in OmpF degradation. Remarkably, given the critical importance attached to such systems for normal E. coli physiology, we find evidence for DegP-mediated OmpF downregulation, gmhB and rseA loss of function mutation in E. coli isolates derived from human infections. Finally, we show that these regulatory mutations enhance the ability of group 1 CTX-M β-lactamase to confer reduced carbapenem susceptibility, particularly those mutations that cause OmpC in addition to OmpF downregulation.