We present a search for long-lived Higgs portal scalars (HPS) and heavy neutral leptons (HNL) decaying in the MicroBooNE liquid-argon time projection chamber. The measurement is performed using data collected synchronously with the NuMI neutrino beam from Fermilab's Main Injector with a total exposure corresponding to 7.01 × 10 20 protons on target. We set upper limits at the 90% confidence level on the mixing parameter |Uµ4| 2 ranging from |Uµ4| 2 < 12.9 × 10 −8 for Majorana HNLs with a mass of mHNL = 246 MeV to |Uµ4| 2 < 0.92 × 10 −8 for mHNL = 385 MeV, assuming |Ue4| 2 = |Uτ4| 2 = 0 and HNL decays into µ ± π ∓ pairs. These limits on |Uµ4| 2 represent an order of magnitude improvement in sensitivity compared to the previous MicroBooNE result. We also constrain the scalar-Higgs mixing angle θ by searching for HPS decays into µ + µ − final states, excluding a contour in the parameter space with lower bounds of θ 2 < 31.3 × 10 −9 for mHPS = 212 GeV and θ 2 < 1.09 × 10 −9 for mHPS = 275 GeV. These are the first constraints on the scalar-Higgs mixing angle θ from a dedicated experimental search in this mass range.