Proteus mirabilis (P. mirabilis) is increasingly recognized as a causative agent of hospital and community acquired infections, including those in long-term care facilities (LTCFs). Previous studies on P. mirabilis from Croatia showed the predominance of TEM-52 extended spectrum beta-lactamase (ESBL) and the emergence of plasmid AmpC beta-lactamases. The aim of the present study was to investigate the evolution and dynamics of cephalosporinases in P. mirabilis from LTCFs and to compare the isolates found in the LTCFs with those from a community setting. A total of 41 isolates of P. mirabilis resistant to third-generation cephalosporins were collected from two nursing homes and from outpatients from March 2015 until September 2017. Antibiotic susceptibility testing was performed by the broth microdilution method. ESBLs and plasmid-mediated AmpC beta-lactamases were detected by phenotypic inhibitor-based tests and by polymerase chain reaction (PCR). Plasmids were characterized by utilizing conjugation and transformation experiments, as well as PCR-based replicon typing. All isolates exhibited high level of resistance to amoxicillin alone and in combination with clavulanic acid, as well as to first-, second- and third-generation cephalosporins. Three isolates tested positive in inhibitor-based test with clavulanic acid, and 38 in Hodge test and combined disk test with phenylboronic acid, indicating the production of ESBLs and plasmid-mediated AmpC beta-lactamases, respectively. Two ESBL-positive organisms yielded amplicons with primers specific for CTX-M beta-lactamase of group 1 and one for TEM. All AmpC-positive organisms were identified by PCR as CMY. CTX-M positive strains harbored IncK plasmid, whereas AmpC-positive strains were negative for known plasmid types. This study demonstrated persistence of CMY beta-lactamases in one LTCF, but also dissemination of the aforementioned resistance determinants to another nursing home and to the community setting. Akin to other studies, there was a trend of cephalosporinase dynamic switch from TEM variants to CTX-M and CMY. Therefore, accurate and swift laboratory identifi cation of cephalosporinase type is becoming pivotal for appropriate choice of treatment.