Mesoscale warm-core rings, known as Loop Current rings (LCRs) reshape the Gulf of Mexico water masses by redistributing large amounts of heat and salt laterally. LCRs also transform water masses via diapycnal mixing, but the mechanisms by which this occurs are poorly measured. Here, we present glider-MicroPod turbulence observations that reveal enhanced mixing below the mixed layer, along the eddy edges, driving the LCR’s heat, salt, and oxygen exchanges. Submesoscale stirring at the LCR’s edge yields interleavings of adjacent water masses, which facilitates double-diffusive mixing that transforms Subtropical Underwater into Gulf Common Water. Our findings highlight the need for ocean models to parameterize double-diffusive mixing processes directly resulting from submesoscale tracer stirring, which may be important at basin scale in the presence of LCRs in the Gulf of Mexico.