Ultraviolet physics typically induces a kinetic mixing between gauge singletswhich is marginal and hence non-decoupling in the infrared. In singletextensions of the minimal supersymmetric standard model, e.g. thenext-to-minimal supersymmetric standard model, this furnishes a well motivatedand distinctive portal connecting the visible sector to any hidden sector whichcontains a singlet chiral superfield. In the presence of singlet kineticmixing, the hidden sector automatically acquires a light mass scale in therange 0.1 - 100 GeV induced by electroweak symmetry breaking. In theories withR-parity conservation, superparticles produced at the LHC invariably cascadedecay into hidden sector particles. Since the hidden sector singlet couples tothe visible sector via the Higgs sector, these cascades necessarily produce aHiggs boson in an order 0.01 - 1 fraction of events. Furthermore,supersymmetric cascades typically produce highly boosted, low-mass hiddensector singlets decaying visibly, albeit with displacement, into the heavieststandard model particles which are kinematically accessible. We studyexperimental constraints on this broad class of theories, as well as the roleof singlet kinetic mixing in direct detection of hidden sector dark matter. Wealso present related theories in which a hidden sector singlet interacts withthe visible sector through kinetic mixing with right-handed neutrinos.