[en] The ground state magnetic dipole moment of ⁹C has been remeasured using the β-NMR technique and our new value of 1.396(3)μ_N is in agreement with the previously measured value for this nuclide. The quenched magnetic moment of ⁹C and the anomalously large isoscalar spin expectation value for the ⁹C-⁹Li T=3/2 mirror pair compare favorably with the results of shell model calculations employing isospin nonconserving (INC) interactions. The ground state magnetic moment of ²¹Mg was also investigated to determine the significance of INC effects in the heavier A=21 T=3/2 mirror system