An Nd:YAG planar waveguide laser has been fabricated by ultra-low-fluence ( $2\times {10}^{12}{\mathrm{cm}}^{-2}$ ) swift heavy-ion irradiation ( $60\mathrm{MeV}$ ${\mathrm{Ar}}^{4+}$ ions). The appearance of the buried waveguiding has been associated with an increased refractive index layer as a consequence of the ion-induced electronic damage. Continuous-wave laser oscillations at $1064.2\mathrm{nm}$ have been observed from the waveguide under $808\mathrm{nm}$ optical excitation, with the absorbed pump power at threshold and laser slope efficiency close to $26\mathrm{mW}$ and 5.9%, respectively.