Disclosed are ionene polymers having antimicrobial activity. "Ionene polymers" as used in this invention are cationic polymers in which a substantial proportion of the atoms providing the positive charge are quaternized nitrogens located in the main polymeric chain or backbone of the polymer rather than in pendant groups. Also disclosed are antimicrobial compositions comprising ionene polymers and methods for treating microbial infections in mammals comprising the step of administering to a mammal, a therapeutically effective amount of at least one antimicrobial composition of the invention. Also disclosed are antimicrobial compositions comprising at least one ionene polymer and methods for preventing, inhibiting or eliminating the growth, dissemination, and/or the accumulation of microorganisms on a susceptible surface (including, but not limited to, the formation of biofilms on a susceptible surface) comprising the step of contacting such surface with a composition of the invention.