Objective: To compare the length of the membranous (functional) urethra in male patients who underwent the male transobturator sling (TOS) for postradical prostatectomy urinary incontinence (PPI). The TOS is in established use for treatment of PPI; however, the precise mechanism of action is unknown. Materials and Methods: This is a prospective case-controlled study on men undergoing male TOS surgery from 2008 to 2014. The comparison arm included patients without incontinence after radical prostatectomy. All participants underwent dynamic magnetic resonance imaging (MRI) at baseline and this was repeated after TOS placement for those who underwent the procedure. Three standardized points were measured using MRI and compared in both groups in addition to clinical measures. Results: Thirty-nine patients were enrolled and 31 patients completed the protocols. The controls (N = 14) had a longer vesicourethral anastomosis to urethra measured at the penile bulb (functional urethral length) distance compared to the pre-TOS group at rest (1.92 cm controls vs 1.27 cm pre-TOS, P =.0018) and at Valsalva (2.13 cm controls vs 1.72 cm pre-TOS, P =.0371). Placement of the sling (N = 17) increased the functional urethral length distance at rest (1.92 cm control vs 1.53 cm post-TOS, P =.09) and at Valsalva (1.94 cm post-TOS vs 2.13 cm control, P =.61), so that the difference was no longer statistically significant. Conclusion: We identified that one possible mechanism in improvement in stress urinary incontinence post-TOS placement is the lengthening of the vesicourethral anastomosis to bulbar-urethra distance. This is the first such study utilizing dynamics MRI in post prostatectomy controls, incontinent pre-TOS, and post-TOS to assess and show these findings.