We measure properties of the 795 nm
3H
6 to
3H
4 transition of a rare-earth-ion-doped waveguide, Tm
3+:Ti
4+:LiNbO
3, at temperatures as low as 800 mK. Coherence and hyperfine population lifetimes up to 117
μs and 2.5 hours, respectively, are observed. They are at least one order of magnitude greater than those measured at 3 K, and match those observed in a bulk Tm
3+:LiNbO
3 crystal under similar conditions. Furthermore, taking advantage of these properties, we prepare a 0.5 GHz-bandwidth atomic frequency comb quantum memory of finesse $>$2 on a vanishing background. Finally, we measure high Rabi frequencies and calculate a transition dipole moment of $\sim
10^{-32}$ C
⋅m, which is equivalent to that of Tm
3+:LiNbO
3. Our results show the suitability of rare-earth-doped waveguides created using industry-standard Ti-indiffusion in LiNbO
3 for on-chip quantum applications.