We report a measurement of the top quark mass using six candidate events for the process $p\overline{p}\ensuremath{\rightarrow}t\overline{t}+\stackrel{\ensuremath{\rightarrow}}{X}{l}^{+}\ensuremath{\nu}{\mathrm{bl}}^{\ensuremath{-}}\overline{\ensuremath{\nu}}\overline{b}+X$, observed in the D0 experiment at the Fermilab $p\overline{p}$ collider. Using maximum likelihood fits to the dynamics of the decays, we measure a mass for the top quark of ${m}_{t}=168.4\ifmmode\pm\else\textpm\fi{}12.3(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}3.6(\mathrm{syst}) \mathrm{Gev}$. We combine this result with our previous measurement in the $t\overline{t}\ensuremath{\rightarrow}l+\mathrm{jets}$ channel to obtain ${m}_{t}=172.1\ifmmode\pm\else\textpm\fi{}7.1 \mathrm{GeV}$ as the best value of the mass of the top quark measured by D0.