We present a combination of measurements of the top quark mass by the D0 experiment in the $\mathrm{lepton}+\text{jets}$ and dilepton channels. We use all the data collected in Run I (1992--1996) at $\sqrt{s}=1.8\text{ }\text{ }\mathrm{TeV}$ and Run II (2001--2011) at $\sqrt{s}=1.96\text{ }\text{ }\mathrm{TeV}$ of the Tevatron $p\overline{p}$ collider, corresponding to integrated luminosities of $0.1\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ and $9.7\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$, respectively. The combined result is: ${m}_{t}=174.95\ifmmode\pm\else\textpm\fi{}0.40(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0ex}{0ex}}0.64(\mathrm{syst})\text{ }\text{ }\mathrm{GeV}=174.95\ifmmode\pm\else\textpm\fi{}0.75\text{ }\text{ }\mathrm{GeV}$.