The effects of hydrostatic pressure and aluminum concentration on the conduction-electron effective Land\'e $g$ factor in semiconductor $\mathrm{Ga}\mathrm{As}\text{\ensuremath{-}}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ quantum wells under in-plane magnetic fields are presented. Numerical calculations of the conduction-electron Land\'e $g$ factor are performed by taking into account the nonparabolicity and anisotropy of the conduction band via the Ogg--McCombe Hamiltonian as well as the effects of aluminum concentration and applied hydrostatic pressure. Theoretical results are given as functions of the aluminum concentration in the ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ barrier, orbit-center position, applied in-plane magnetic field, hydrostatic pressure, and quantum-well width, and found in good agreement with experimental measurements in $\mathrm{Ga}\mathrm{As}\text{\ensuremath{-}}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ quantum wells for various values of the aluminum concentration $x$ in the absence of hydrostatic pressure.