We use high-resolution synchrotron x-ray and neutron diffraction to study the geometrically frustrated triangular lattice antiferromagnet $\mathrm{Cu}\mathrm{Fe}{\mathrm{O}}_{2}$. On cooling from room temperature, $\mathrm{Cu}\mathrm{Fe}{\mathrm{O}}_{2}$ undergoes two antiferromagnetic phase transitions with incommensurate and commensurate magnetic order at ${T}_{N1}=14\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${T}_{N2}=11\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively. The occurrence of these two magnetic transitions is accompanied by second- and first-order structural phase transitions from hexagonal to monoclinic symmetry. Application of a $6.9\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ magnetic field lowers both transition temperatures by $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and induces an additional incommensurate structural modulation in the temperature region where the field-driven ferroelectricity occurs. These results suggest that a strong magneto-elastic coupling is intimately related to the multiferroic effect.