  By applying mirror symmetry to D-branes in a Calabi-Yau geometry we shed
light on a $G_2$ flop in M-theory relevant for large $N$ dualities in ${\cal
N}=1$ supersymmetric gauge theories. Furthermore, we derive superpotential for
M-theory on corresponding $G_2$ manifolds for all A-D-E cases. This provides an
effective method for geometric engineering of ${\cal N}=1$ gauge theories for
which mirror symmetry gives exact information about vacuum geometry. We also
find a number of interesting dual descriptions.
