  We present the first measurement of the time-integrated mixing probability
$\chi_d$ using Belle II data collected at a center-of-mass (CM) energy of 10.58
GeV, corresponding to the mass of the $\Upsilon$(4S) resonance, with an
integrated luminosity of $34.6 {\rm fb}^{-1}$ at the SuperKEKB $e^+ e^-$
collider. We reconstruct pairs of B mesons both of which decay to semileptonic
final states. Using a novel methodology, we measure $\chi_d = 0.187 \pm 0.010
\text{ (stat.)} \pm 0.019 \text{ (syst.)}$, which is compatible with existing
indirect and direct determinations.
