The dynamics of transverse mode formation time in positive-branch unstable resonators with low magnification (M = 1.32) and typical Fresnel numbers (between 279 and 2700) used in high-energy lasers has been investigated. The objective was to observe the far-field intensity distribution from the onset of pumping to the settled steady-state diffraction-limited pattern. The research consisted of an experimental and numerical analysis. The experimental measurements of mode formation time were with a pulsed XeF laser having centered resonator optics. Mode formation was deduced from observation of the far field. The analytical effort consisted of a fast Fourier transform wave optics code which modeled the far-field pattern evolution. Results agree with temporal mode formation geometrical predictions of Ananev (1975) when laser saturation is achieved around this time.