The host structure and reversible lithium insertion and extraction of an intercalation compound, TiNb$\mathrm{<mrow\; data-mjx-texclass="ORD">24</mrow>}$O$\mathrm{<mrow\; data-mjx-texclass="ORD">62</mrow>}$, are described. Neutron diffraction, applied for the first time to TiNb$\mathrm{<mrow\; data-mjx-texclass="ORD">24</mrow>}$O$\mathrm{<mrow\; data-mjx-texclass="ORD">62</mrow>}$, allowed an accurate refinement of the complex block superstructure, particularly with respect to the oxygen sublattice. Analysis of the transition-metal sites revealed significant cation ordering in the mixed-metal oxide. Electrochemical analysis demonstrated highly reversible lithium intercalation with ca. 190 mA·h·g$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) after 100 cycles ($C$/10 rate, 3 months). The effect of the potential window on the capacity, polarization, and reversibility was carefully examined; a minimum voltage limit of 1.1-1.2 V is critical for efficient and reversible cycling. The galvanostatic intermittent titration technique revealed three solid-solution regions, with different lithium diffusivities, in addition to the two-phase plateau that was clearly observed in the $V$ versus $Q$ discharge/charge profile. Lithium-ion diffusion decreases by over 3 orders of magnitude from the dilute lithium limit early in the discharge to the lithium-stuffed phase Li$\mathrm{<mrow\; data-mjx-texclass="ORD">37.5(1.0)</mrow>}$TiNb$\mathrm{<mrow\; data-mjx-texclass="ORD">24</mrow>}$O$\mathrm{<mrow\; data-mjx-texclass="ORD">62</mrow>}$. Nevertheless, prior to lithium stuffing, TiNb$\mathrm{<mrow\; data-mjx-texclass="ORD">24</mrow>}$O$\mathrm{<mrow\; data-mjx-texclass="ORD">62</mrow>}$ possesses intrinsically rapid lithium-ion kinetics, as demonstrated by the high-rate performance in thick films of ca. 10 μm particles when interfaced with a carbon-coated aluminum foil substrate. The TiO$2$·Nb$2$O$5$ phase diagram is examined and electrochemical results are compared to related superstructures of crystallographically sheared blocks of octahedra in the TiO$2$·Nb$2$O$5$ homologous series including the H-Nb$2$O$5$ end member.