The $N$-representability problem for non-collinear spin-polarized densities was left open since the pioneering work of von Barth and Hedin setting up the Kohn-Sham density functional theory for magnetic compounds. In this letter, we demonstrate that, contrarily to the non-polarized case, the sets of pure and mixed state $N$-representable densities are different in general. We provide a simple characterization of the latter by means of easily checkable necessary and sufficient conditions on the components $\rho^{\alpha \beta} (\br)$ of the spin-polarized density.