In this work, the ground and excited states of ThBe and AcBe were investigated by performing high-level multireference and single-reference coupled-cluster quantum chemical calculations with large correlation consistent basis sets. Full potential energy curves (PECs), chemical bonding patterns, energetics, spectroscopic parameters (T_e, r_e, ω_e, and ω_ex_e), and spin–orbit effects of 13 and 8 electronic states of ThBe and AcBe, respectively, are reported. The ground electronic states of ThBe and AcBe are single-reference 1³Σ⁻ (1σ²2σ²1π²) and 1²Π (1σ²2σ²1π¹), respectively, and originate from their corresponding ground state fragments. The chemical bonding of ThBe (1³Σ⁻) and AcBe (1²Π) are π-dative in character and are formed by d-electron transfers from Th/Ac to the empty 2p_x and 2p_y of the Be atom. The electron populations of the f-orbitals of both ThBe (1³Σ⁻) and AcBe (1²Π) are minor which exhibit their “transition-metal-like” nature. The estimated bond energies of the spin–orbit ground states of ThBe (1³Σ⁻_0⁺) and AcBe (1²Π_1/2) are 12.79 and 11.02 kcal mol⁻¹, respectively. Finally, the bond energy of ThBe was used to estimate its heat of formation ΔH⁰_f(298 K) of 869.61 ± 6 kJ mol⁻¹.