High carrier density quantum wells embedded within a Mott insulating matrix present a rich arena for exploring unconventional electronic phase behavior ranging from non-Fermi-liquid transport and signatures of quantum criticality to pseudogap formation. Probing the proposed connection between unconventional magnetotransport and incipient electronic order within these quantum wells has however remained an enduring challenge due to the ultra-thin layer thicknesses required. Here we address this challenge by exploring the magnetic properties of high-density SrTiO₃ quantum wells embedded within the antiferromagnetic Mott insulator SmTiO₃ via muon spin relaxation and polarized neutron reflectometry measurements. The one electron per planar unit cell acquired by the nominal d⁰ band insulator SrTiO₃ when embedded within a d¹ Mott SmTiO₃ matrix exhibits slow magnetic fluctuations that begin to freeze into a quasistatic spin state below a critical temperature T^*. The appearance of this quasistatic well magnetism coincides with the previously reported opening of a pseudogap in the tunneling spectra of high carrier density wells inside this film architecture. Our data suggest a common origin of the pseudogap phase behavior in this quantum critical oxide heterostructure with those observed in bulk Mott materials close to an antiferromagnetic instability.