A solid unobtrusive fiber-optic sensor based on modal interference in dual mode fiber (DMF) embedded in a thin mat is proposed and investigated theoretically and experimentally, realizing real-time vital signs and activity monitoring. The whole sensing setup only consists of one laser diode, one section of DMF, one photo-detector and few standard single mode fibers (SMFs). The DMF fiber is sandwiched into the SMFs using core-offset splicing, i.e., SMF-DMF-SMF structure. The optimal value of the core offset is numerically simulated and experimentally validated. It has to be noted that all the fibers used in the experiment are with 900-μm protection jacket, which can effectively avoid fiber breaking. The proposed sensor is both investigated in different postures and different subjects. And the results of vital signs monitoring performance show great consistency compared with commercial monitors. Besides, the activity monitoring is also addressed with one-second discrimination resolution, identifying three basic states, i.e., on bed, body movement and off bed. It is easy to fabricate and shows great potential in future non-invasive vital signs and activity monitoring applications. © 2021 Elsevier Inc.