Ultrafast laser-based shadowgraphic-interferometric imaging of a high-density plasma generated by intense laser-matter interactions is a key diagnostic technique providing information on the evolution and ultrafast dynamics of such a plasma state. However, shadowgraphic techniques have received modest attention in diagnosing low-density gaseous plasmas similar to those used in laser-plasma wakefield accelerators. Here, by using an ultrafast 30 fs probe laser beam, we derived a correlation between the plasma dynamics and electron beam characteristics in a laser-plasma acceleration experiencing ionization injection in the plasma of a He-N2 gas mixture. The ultrafast transverse expansion dynamics of the laser-plasma channel front tends to be faster at higher nitrogen concentrations, which leads to an increase in the divergence angle and yield (charge) of the 100−200 MeV electron beams generated in the current experiment. Computer simulation shows a distinct difference between the dynamics of the self-injection and that of the ionization-injection processes and supports the experimental observations.