This paper reports on an experimental study involving examination of the effects of an interfacial gap in the range of 0–0.3 mm on the weld strength and weld appearance of lap welds produced by laser welding. AISI 304 stainless steel and AZ31 magnesium alloy joints were processed separately using an IPG YLR-1000 fiber laser. The weld joint strengths were mechanically tested by both tensile shear and T-peel testing. The test results indicate that an adequate amount of interface gap significantly improves weld strength and surface appearance for both AISI 304 stainless steel and the AZ31 magnesium alloy. In addition, it is shown that porosity formation in laser welding AZ31 magnesium alloy can be mitigated by adding an optimal interface gap. Finding the optimum gap size can maximize the quality of lap welds within the constraints of the laser equipment.