A search for non-resonant Higgs boson pair production, as predicted by the Standard Model, is presented, where one of the Higgs bosons decays via the channel and the other via one of the channels. The analysis selection requires events to have at least two -tagged jets and exactly two leptons (electrons or muons) with opposite electric charge in the final state. Candidate events consistent with Higgs boson pair production are selected using a multi-class neural network discriminant. The analysis uses 139 fb of collision data recorded at a centre-of-mass energy of 13 TeV, by the ATLAS detector at the Large Hadron Collider. An observed (expected) upper limit of 1.2 () pb is set on the non-resonant Higgs boson pair production cross-section at 95% confidence level, which is equivalent to 40 () times the value predicted in the Standard Model.