Throughout nature, both free radicals and transient radical reaction intermediates are vital to biological function. Coenzyme B12 is a case in point. This organometallic cofactor generates a radical pair upon activation in its dependent enzymes by substrate binding and following photolysis. The resulting cob(II)alamin / 5’- deoxyadenosyl radical pair has unusual magnetic properties that present a challenge to detailed investigation at ambient temperatures. Here, we use femtosecond transient absorption spectroscopy adapted for magnetic field exposure to reveal that the spin dynamics of the B12 radical pair are sufficiently fast for magnetic field effects to be observed on the ultrafast reaction kinetics. Moreover, the large difference in g-values between the radicals of the pair mean that effects of the Δg mechanism are observed for the first time for a radical pair system exposed to magnetic fields below 1 T. Spin dynamic simulations allow a value of the cob(II)alamin radical g-value (2.105) at ambient temperature to be extracted and, because the spin dynamic timescale is faster than the diffusional rotation of the cob(II)alamin radical, the observed value corresponds to the anisotropic g|| value for this radical.