We present our results from time-domain THz spectroscopy measurements of thin films of mixed-valent YbAl3 and its structural analogue LuAl3. Combined with Fourier transform infrared (FTIR) spectroscopy, the extended Drude formalism is utilized to study the quasiparticle scattering rate and effective masses in YbAl3. We find that LuAl3 demonstrates conventional Drude transport whereas at low temperatures YbAl3 demonstrates a renormalized Drude peak and a mid-infrared (MIR) peak in the conductivity, indicative of the formation of a mass-enhanced Fermi liquid (FL). In YbAl3 the extended Drude analysis demonstrates consistency with FL behavior below the FL coherence temperature T∗< 40 K with the scattering rate following T2 proportionality and a moderate mass enhancement. Despite not observing a clear ω2 Fermi liquid-like frequency dependence the evidence is consistent with a moderate mass Fermi liquid, albeit one with a smaller mass than observed in single crystals. The extended Drude analysis also demonstrates a slow crossover between the FL state and the normal state above the T∗ in YbAl3, indicative of incoherent hybridization effects persisting to high temperatures. © 2021, The Author(s), under exclusive licence to EDP Sciences, SIF and Springer-Verlag GmbH Germany, part of Springer Nature.