We report magnetic and spectroscopic observations and modeling of the Of?p star HD 148937 within the context of the MiMeS LP at the CFHT. Thirty-two high signal-to-noise ratio circularly polarised (Stokes V) spectra and 13 unpolarised (Stokes I) spectra of HD 148937 were acquired in 2009 and 2010. A definite detection of a Stokes V Zeeman signature is obtained in the grand mean of all observations (in both LSD mean profiles and individual spectral lines). The longitudinal magnetic field inferred from the Stokes V LSD profiles is consistently negative, in contrast to the essentially zero field strength measured from the diagnostic null profiles. A period search of equivalent width measurements confirms the previously-reported 7.03 d variability period. The variation of equivalent widths is not strictly periodic: we present evidence for evolution of the amount or distribution of circumstellar plasma. Interpreting the 7.03 d period as the stellar rotational period within the context of the ORM, we have phased the equivalent widths and longitudinal field measurements. The longitudinal field measurements show a weak sinusoidal variation of constant sign, with extrema out of phase with the H{alpha} variation by about 0.25 cycles. The inferred magnetic configuration confirms the suggestion of Naze et al (2010), who proposed that the weaker variability of HD 148937 as compared to other members of this class is a consequence of the stellar geometry. Based on the derived magnetic properties and published wind characteristics, we find a wind magnetic confinement parameter etaast simeq 20 and rotation parameter W = 0.12, supporting a picture in which the Halpha emission and other line variability have their origin in an oblique, rigidly rotating magnetospheric structure resulting from a magnetically channeled wind. (Abridged.)
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