Two extended Wells–Dawson arsenomolybdates, (imi)₂[{Cu^I(imi)₂}₂{Na(imi)₂}{As^IIIAs₂^VMo₁₈O₆₂}]·2H₂O (1) and {Cu^I_0.5(trz)}₆ [{Cu^I_0.5(trz)}₆(As₂Mo₁₈O₆₂)] (2), (imi = iminazole; trz = 1,2,3-triazole), are hydrothermally synthesized and extensively characterized. Compounds 1 and 2 contain a fully oxidized Dawson {As₂Mo₁₈O₆₂}⁶⁻ (abbreviated as {As₂Mo₁₈}) anion as the parent cluster unit. Compound 1 is a symmetric mono-arsenate capped anion cluster modified by two half occupancy {AsO₃} units, which represents a new capping mode for Dawson clusters. The two adjacent mono-arsenate capped structures are linked by two {Na(imi)₂} bridges to form a dimeric cluster, which, as a tetra-dentate ligand, further connects with adjacent dimers by four {Cu(imi)₂} linkers to form an infinite ladder shaped chain. In compound 2, each classical Dawson cluster links six adjacent analogs by six {Cu_0.5(trz)₂} fragments forming an unprecedented 3D network via a unique belt-to-belt staggered pattern. This structure can be simplified as a 2,6-connected 3-D framework with {8¹²·12³}{8}₃ topology. Compounds 1 and 2 represent the first 1-D and 3-D assemblies of Dawson-type arsenomolybdates, respectively. They display unique electrocatalytic behavior for the oxidation of ascorbic acid (AA) and reduction of NO₂⁻ as well as good photocatalytic properties for the degradation of the difficult-to-degrade dye AP under UV light.