Canadian Shield brines are characterized by high concentrations of both stable iodine and 129I, the former reflecting its concentrated but diagenetically modified seawater origin, and the latter its long subsurface residence time within its host shield rocks. Stable iodine concentrations in the Yellowknife brine are as high as 18 mg/L, requiring the contribution of iodine from an external marine reservoir. This reservoir likely comprised organic-rich marine sediments in the basin where the brine was produced. Diagenesis of these sediments released iodine to the brine during its infiltration into the subsurface. 129I concentrations in the Yellowknife brine are as high as 3.4 × 108 atom/L and are due to the fission of 238U in the rock matrix and subsequent diffusion of the radiogenic 129I into the fracture fluids. This concentration is close to the predicted secular equilibrium concentration for a fluid in basaltic rock with a uranium concentration of 1 mg/kg after the residence time of ∼80 m.y. required for secular equilibrium has been attained. This time period is interpreted as the minimum residence time of the Yellowknife brine in the shield, but it may be much greater, possibly dating back to Middle Devonian time when most of the present-day shield was covered by seawater. Similarly high stable iodine and 129I concentrations measured in the Sudbury brine support a generic, ancient marine hypothesis for the origin of shield brines. Stable iodine and 129I concentrations in the shallower mine waters are the result of mixing between the brine and low-iodine modern meteoric recharge that is enriched in 129I due to global fallout from nuclear fuel waste reprocessing activities.