We report on the measurement of lifetimes of excited states in the near-mid-shell nuclei 164,166 Dy using the gamma-ray coincidence fast-timing method. The nuclei of interest were populated using reactions between an 18 O beam and a gold-backed isotopically enriched 164 Dy target of thickness 6.3 mg/cm 2 at primary beam energies of 71, 76, and 80 MeV from the IPN-Orsay laboratory, France. Excited states were populated in 164 Dy, 166 Dy, and 178,179 W following Coulomb excitation, inelastic nuclear scattering, two-neutron transfer, and fusionevaporation reaction channels respectively. Gamma rays from excited states were measured using the ν-Ball high-purity germanium (HPGe)-LaBr 3 hybrid γ-ray spectrometer with the excited state lifetimes extracted using the fast-timing coincidence method using HPGe-gated LaBr 3-LaBr 3 triple coincident events. The lifetime of the first I π = 2 + excited state in 166 Dy was used to determine the transition quadrupole deformation of this neutron-rich nucleus for the first time. The experimental methodology was validated by showing consistency with previously determined excited state lifetimes in 164 Dy. The half-lives of the yrast 2 + states in 164 Dy and 166 Dy were 2.35(6) and 2.3(2) ns, respectively, corresponding to transition quadrupole moment values of Q 0 = 7.58(9) and 7.5(4) eb, respectively. The lifetime of the yrast 2 + state in 166 Dy is consistent with a quenching of nuclear quadrupole deformation at β ≈ 0.35 as the N = 104 mid-shell is approached.