Magnetic treatment is a method for improving the cold flowability of waxy oils. Previous studies have predominantly focused on the viscosity reduction resulted from the treatment, with the durability of the magnetic effect neglected, which is crucial for pipeline transportation of the treated crude oil. Therefore, this study focuses on the durability and its mechanism of the magnetic effect of a waxy crude oil under static, low shear, and high shear conditions. A viscosity reduction of 15.7% was achieved under the magnetic treatment condition of the magnetic treatment temperature at 52 °C, magnetic field strength at 0.1 T, and a duration of 1 min. However, the magnetic effect gradually diminished with time elapsing and disappeared in 9 h under static conditions. Shear was found to be beneficial to the preservation of the effect, and a correlation between the viscosity of the sheared treated-oil and the energy dissipation of the shear was found. Microscopic observations, impedance measurements, and x-ray diffraction analysis revealed that exposure to a magnetic field might disperse the charged particles, i.e., resins and asphaltenes, in the crude oil, facilitating their adsorption on the wax particle surfaces, thus enhancing electrostatic repulsion among wax particles and resulting in viscosity reduction. The desorption of the adsorbed resins and asphaltenes from the wax particles and reaggregation lead to the gradual diminishment of the viscosity reduction. Shear might inhibit this reaggregation and thus contribute to the durability of the viscosity reduction.