The new Y3MnAu5 intermetallic phase is obtained from the arc-melted elements in virtually quantitative yields after annealing at 1000 degrees C for similar to 3 d. Its remarkable structure [rhombohedral, R (3) over bar, Z = 6; a = 8.489(1) angstrom, c = 18.144(2) angstrom] consists of a 2:1 cubic-close-packed intergrowth between edge-shared Mn-centered Au rhombohedra (Mn@Au-8) with gold-centered antiprismatic (Au@Y-6) clusters via a common gold network. Magnetic susceptibility (chi) data for Y3MnAu5 were fitted by a Curie-Weiss law. The Curie constant indicates a large effective moment corresponding to nearly localized Mn spins S = 5/2, and the Weiss temperature demonstrates the dominance of ferromagnetic (FM) interactions. An antiferromagnetic (AFM) transition at T-N = 75 K and a possible spin reorientation transition at 65 K were observed. Analysis of the chi data for T < T-N suggests a planar noncollinear helical AFM structure that arises from competing AFM interactions between FM-aligned layers of spins in the ab-plane with a turn angle of 69 degrees between the spins along the helix c-axis. A magnetic field-induced spin flop transition is observed below T-N. Spin-polarized LMTO-LSDA calculations indicate an similar to 2 eV splitting of the Mn 3d states and a metallic ground state, and their COHP analyses demonstrate that similar to 81% of the total Hamilton populations originate from heteroatomic polar Y-Au and Mn-Au bonding. The Mn 3d, Y 4d, and Au 5d characteristics are remarkably diverse: localized and magnetically polarized for Mn; reducing and cationic for Y; and relativistically strongly bonded and oxidizing for Au, bonding of the latter two being broadly delocalized.