The size-dependent electronic, structural, and. magnetic properties of Mn-doped gold clusters have been systematically investigated by using relativistic all-electron density functional theory with generalized gradient approximation. A number of new isomers are obtained for neutral MnAun (n = 1-16) dusters to probe the structural evolution. The two-dimensional (2D) to three-dimensional (3D) transition occurs in the size range n = 7-10 with manifest structure competitions. From size n = 13 to n = 16, the MnAun prefers a gold cage structure with Mn atom locating at the center. The relative stabilities of the ground-state MnAun dusters show a pronounced odd-even oscillation with the number of Au atoms. The magnetic moments of MnAun dusters vary from 3 mu(B) to 6 mu(B) with the different duster size, suggesting that nonmagnetic Au-n dusters can serve as a flexible host to tailor the dopant's magnetism, which has potential applications in new nanomaterials with tunable magnetic properties.