We present the results of combined laser spectroscopy and nuclear magnetic resonance studies of Mg-21. The nuclear ground-state spin was measured to be I = 5/2 with a magnetic moment of mu = -0.983(7)mu(N). The isoscalar magnetic moment of the mirror pair (F-21, Mg-21) is evaluated and compared to the extreme single-particle prediction and to nuclear shell-model calculations. We determine an isoscalar spin expectation value of <sigma > = 1.15(2), which is significantly greater than the empirical limit of unity given by the Schmidt values of the magnetic moments. Shell-model calculations taking into account isospin non-conserving effects, are in agreement with our experimental results. (C) 2009 Elsevier B.V. All rights reserved.