We investigated a series of mono-niobium carbide clusters, NbCn- (n = 2-7), using anion photoelectron spectroscopy. Vibrationally resolved photoelectron spectra were observed for NbC2- and NbC3-, which were both shown to have cyclic C-2v structures. Two isomers were observed for NbC4- and NbC5-. The weak and low electron binding energy isomers were shown to be cyclic structures forming a series with NbC2- and NbC3-, and all have similar and low electron binding energies. The main isomers of NbC4- and NbC5-, which possess much higher electron binding energies, were shown to be due to linear structures, which form a series with NbC6- and NbC7-. All the linear NbCn-clusters were observed to have high electron binding energies and exhibit an even-odd alternation, similar to that observed for pure linear carbon clusters in the same size range. A cyclic to linear structural transition was thus observed for the NbCn- clusters from NbC3- to NbC4-, with the cyclic structures favored for the smaller clusters and the linear isomers favored for the larger clusters.