  Decay spectroscopy of the long-lived states in $^{186}$Tl has been performed
at the ISOLDE Decay Station at ISOLDE, CERN. The $\alpha$ decay from the
low-spin $(2^-)$ state in $^{186}$Tl was observed for the first time and a
half-life of $3.4^{+0.5}_{-0.4}$ s was determined. Based on the $\alpha$-decay
energy, the relative positions of the long-lived states were fixed, with the
$(2^-)$ state as the ground state, the $7^{(+)}$ state at 77(56)~keV and the
$10^{(-)}$ state at 451(56) keV. The level scheme of the internal decay of the
$^{186}$Tl($10^{(-)}$) state ($T_{1/2} = 3.40(9)$ s), which was known to decay
solely through emission of 374 keV $\gamma$-ray transition, was extended and a
lower-limit for the $\beta$-decay branching $b_\beta > 5.9(3)\%$ was
determined. The extracted retardation factors for the $\gamma$ decay of the
$10^{(-)}$ state were compared to the available data in neighboring odd-odd
thallium isotopes indicating the importance of the $\pi d_{3/2}$ shell in the
isomeric decay and significant structure differences between $^{184}$Tl and
$^{186}$Tl.
