This study developed theoretical equations for the estimation of the factor of safety (FS) for local instability of a wedge-shaped double body (WSDB) in soil-nail–reinforced slopes. These equations were used to perform sensitivity analysis by varying soil slope and nail parameters to assess their impact on FS. The results infer that cohesion, friction angle and slope inclination largely control the instability of WSDB between soil nails, while soil thickness, nail spacing, and nail inclination have little or no effect on FS. Finite element analysis was also performed and the results were compared with those obtained from the developed equations. It could be inferred that the FS estimated from the numerical analysis matched well with theoretical estimates when varying friction angle, slope inclination of soil and nail inclination. However, the numerical analysis portrays higher though conservative values with increase in cohesion, as compared to theoretical predictions.