Here, the structure-dependent electronic, thermal, and transport properties of nanostructured thallium telluride (Tl _8 Pb _x Sb _2− _x Te _6 ) through controlled variation in Pb and Sb (x = 1.96, 1.97, 1.98, 1.99) concentrations have been investigated. In the temperature and concentration-dependent electrical conductivity measurements, the highest electrical conductivity 131.96 × 10 ^3 ${{\rm{\Omega }}}^{-1}{m}^{-1}$ at 300 K was measured for x = 1.99 and the maximum observed Seebeck value for the optimized Tl _8 Pb _1.96 Sb _0.04 Te _6 nanoparticles was 110.7 μ V/K at 550 K. Such an increased value of the Seebeck coefficient led to the achievement of a significantly improved high-power factor, which was found to be increasing with temperature and decreasing with the increase of Pb concentration. The density functional theory calculations performed for Pb and Sb co-doped Tl _5 Te _3 resulted in the enhanced σ _e and S with a significant reduction in electronic thermal conductivity ( κ _e ) and is found consistent with experimentally measured κ _e . The highest ZT = 0.35 and 0.18 were recorded experimentally and theoretically for Pb and Sb co-doped in Tl _5 Te _3 nanoparticles.