Geosynthetic-reinforced and column-supported (GRCS) embankment is a practical and cost-effective technique for reinforcing soft soils. Internal stability of the column-supported embankment, which is engaged in shearing and bending failure modes, is one of the key issues that geotechnical engineers encounter in the design. To deal with this problem, numerical models are currently used. In this study, a simplified model has been developed for predicting the potential of internal instability in GRCS embankments. The validity of the deterministic model was verified by comparing the predicted safety factors with experimental and numerical observations. The findings show that the proposed model has the potential to accurately evaluate the internal stability of GRCS embankments. Moreover, probabilistic approaches were also used to undertake the reliability and sensitivity studies of column-supported embankments. The bending moment failure mode has a larger impact on the stability of the GRCS embankment than the shear failure mode. It is also found that the safety factor of shearing and bending failure modes is strongly influenced by the undrained soft soil shear strength. Furthermore, the reliability study demonstrates that the embankment height and the number of column rows are important factors in determining the efficiency of GRCS embankments in resisting internal instability.