Magnetometer data acquired by the MESSENGER spacecraft in orbit about Mercury permit the separation of internal and external magnetic field contributions. The global planetary field is represented as a southward-directed, spin-aligned, offset dipole centered on the spin axis. Positions where the cylindrical radial magnetic field component vanishes were used to map the magnetic equator and reveal an offset of 484 +/- 11 kilometers northward of the geographic equator. The magnetic axis is tilted by less than 3 degrees from the rotation axis. A magnetopause and tail-current model was defined by using 332 magnetopause crossing locations. Residuals of the net external and offset-dipole fields from observations north of 30 degrees N yield a best-fit planetary moment of 195 +/- 10 nanotesla-R(M)(3), where R(M) is Mercury's mean radius.