Recent results from a measurement campaign studying the isomerism in neutron-deficient Tl isotopes are presented. The measurements make use of a nuclear spectroscopy setup coupled to the high-resolution Penning-trap mass spectrometer ISOLTRAP at CERN's radioactive ion-beam facility ISOLDE. The mass values of ${}^{190,194}$Tl are improved and a mass-spin-state assignment is carried out. An additional mass measurement of the grandparent nuclide ${}^{198}$At allows the deduction of the spin-state ordering in ${}^{190}$Tl. As a result, the excitation energies of the isomers in both Tl isotopes are determined for the first time to ${E}_{\mathrm{ex}}{(}^{194}\text{Tl})=260(15)$ keV and ${E}_{\mathrm{ex}}{(}^{190}\text{Tl})=89(12)$ keV. Furthermore, this allows anchoring of the ground-state and isomer masses of ${}^{194}$Bi, ${}^{202}$Fr, and ${}^{206}$Ac, which are linked by two independent $\ensuremath{\alpha}$-decay chains.