The forward-backward asymmetry in Drell--Yan production and the effective leptonic electroweak mixing angle are measured in proton-proton collisions at $\sqrt{s} = 13$\TeV, collected by the CMS experiment and corresponding to an integrated luminosity of 138\fbinv. The measurement uses both dimuon and dielectron events, and is performed as a function of the dilepton mass and rapidity. The unfolded angular coefficient \afour is also extracted, as a function of the dilepton mass and rapidity.Using the CT18Z set of parton distribution functions, we obtain $\sineff = 0.23157 \pm 0.00031$, where the uncertainty reflects the statistical and systematic (experimental and theoretical) contributions.The measured value agrees with the standard model fit result to global experimental data.The measured value agrees with the standard model fit result to global experimental data.This is the most precise \sineff measurement at a hadron collider, with a precision comparable to the results obtained at LEP and SLD.