The changes of the surface microrelief and microhardness $HV$ of the surface layers of the two-phase Cu–39Zn–1Pb brass are experimentally studied after high-frequency impact treatment induced by ultrasound (ultrasonic impact treatment—UIT) in the air and inert (argon gas) environments. As shown, the argon-UIT leads to the substantial decrease in the roughness parameters of the modified surface. Conversely, the air-UIT results in the formation of more developed surface microrelief. An increase in the surface $HV$ of the Cu–39Zn–1Pb brass samples after UIT in argon and in air reaches $\cong$180% ($HV_{100}$ = 2.24 GPa) and $\cong$220% ($HV_{100}$ = 2.76 GPa), respectively. In both cases the strengthening is registered on a depth up to $\cong$1 mm. Instrumental indentation measurements of the samples after UIT in the air and argon show the increased hardness ($H_{IT}$ = 3.236 GPa and $H_{IT}$ = 2.469 GPa) well correlating to the $HV$ data, the decreased plasticity characteristic $\delta_{A}$ and Young modulus $E$ decreased from initials states (107 GPa) to 94 GPa and 91 GPa, respectively. Taking into account the SEM data and X-ray structural and phase analysis, the main factors affecting the strain hardening extents of the Cu–39Zn–1Pb brass surface layers in different environments are determined: growth of $\alpha$-phase amount, reorientation of $\alpha$-phase grains, and compressive residual stresses.