(1) Background: The purpose of our prospective, single-blinded, randomized, sham-controlled study was to investigate the effect of the additional extracorporeal magnetic stimulation (ExMI) to pharmacological treatment in overactive bladder syndrome (OAB) in women. (2) Methods: We recruited 56 women with OAB, who were allocated into two study groups: the active group received mirabegron 50 mg daily and a total of 16 sessions of ExMI in 8 weeks, whereas the sham group received mirabegron 50 mg daily and sham stimulation following the same treatment protocol. Treatment success was evaluated after 4 and 8 weeks. (3) Results: Both groups experienced significant reduction in daytime urinary frequency, nocturia, and number of weekly incontinence episodes after 8 weeks. There were no statistically significant differences in end-point daytime urinary frequency and nocturia between groups. However, the overall average reduction rate in weekly number of incontinence episodes was 43.7% in treatment group and 24.2% in the control group. The number of urinary incontinence episodes in the treatment and control group was reduced for 3.8 ± 11.8 vs. 2.5 ± 4.3 episodes at week 4 and additional 3.3 ± 6 vs. 0.4 ± 3.2 episodes at week 8, respectively (p = 0.013). Moreover, IIQ-7 score showed a significantly greater score reduction and patients’ evaluated improvement of symptoms was higher in the active group. (4) Conclusions: The addition of ExMI to mirabegron in OAB treatment further improves the weekly incontinence episode reduction rate and also leads to grater improvement in symptoms.