  Millimeter-wave superconducting devices offer a platform for quantum
experiments at temperatures above 1 K, and new avenues for studying
light-matter interactions in the strong coupling regime. Using the intrinsic
nonlinearity associated with kinetic inductance of thin film materials, we
realize four-wave mixing at millimeter-wave frequencies, demonstrating a key
component for superconducting quantum systems. We report on the performance of
niobium nitride resonators around 100 GHz, patterned on thin (20-50 nm) films
grown by atomic layer deposition, with sheet inductances up to 212 pH/square
and critical temperatures up to 13.9 K. For films thicker than 20 nm, we
measure quality factors from $1$-$6\times 10^4$, likely limited by two-level
systems. Finally we measure degenerate parametric conversion for a 95 GHz
device with a forward efficiency up to +16 dB, paving the way for the
development of nonlinear quantum devices at millimeter-wave frequencies.
