Laminar-to-turbulent transition in shear flow can be complicated and may follow many possible routes. The scenario based on the soliton/like coherent structure (SCS) [Lee and Wu, “Transition in wall-bounded flows”, Appl. Mech. Rev. 61, 030802, (2008)] has drawn much attention to the role of hairpin vortices and their origin. This letter reports the existence of such SCSs in a compressible mixing layer and explores how their interactions with the background shear flow generate the streamwise vorticity. Lagrangian particle tracking based on direct numerical simulation data is employed to trace the dynamic evolution of such structures. Evidence shows that localized three-dimensional waves travel coherently during the early transition stage, in good consistence with the behavior of SCSs in boundary layer transition. Through visualization of disturbance amplification and statistical analysis of locally distributed high-strain rate regions, it is suggested that SCSs originate from local oblique resonance effects in mixing layers, with the motion of these structures along the mean shear flow contributing to the generation of hairpin vortices.