The present work is aimed at the elaboration of the model of magnetic properties and magnetic relaxation in the mononuclear [Pc ₂Tb] ^-TBA ⁺ complex that displays single-molecule magnet properties. We calculate the Stark structure of the ground ⁷F ₆ term of the Tb ³⁺ ion in the exchange charge model of the crystal field, taking account for covalence effects. The ground Stark level of the complex possesses the maximum value of the total angular momentum projection, while the energies of the excited Stark levels increase with decreasing |M _J| values, thus giving rise to a barrier for the reversal of magnetization. The one-phonon transitions between the Stark levels of the Tb ³⁺ ion induced by electron-vibrational interaction are shown to lead to magnetization relaxation in the [Pc ₂Tb] ^-TBA ⁺ complex. The rates of all possible transitions between the low-lying Stark levels are calculated in the temperature range 14 K<T < 40 K. With the purpose of calculation of the temperature dependence of the relaxation time of magnetization, we solve the set of master equations for the populations of the Stark levels. The relaxation time is shown to diminish from 3.2 × 10 ^-2 s to 1.52 × 10 ^-4 s as the temperature increases from 27 K to 40 K. The obtained values of the relaxation time are in satisfactory agreement with the observed ones. The developed model also provides satisfactory description of the dc-magnetic data and paramagnetic shifts.