The use of conductive fabrics (CFs) in the design of wearables for joint sensing has recently received much interest in a wide range of applications such as robotics, rehabilitation, personal wellness, and sports. However, one key limitation in the existing measurement approach is that the user’s anthropometric information is required to relate the joint parameters to the CF sensor strain reading. This paper seeks to address this limitation by evaluating a new wearable device concept that comprises a CF strain–voltage sensor embedded as part of an inverted slider-crank (ISC) mechanism for joint extension sensing. This benefits from not requiring anthropometric information from the user to relate the joint parameters to the fabric strain readings, as opposed to an existing design. We first characterize the electromechanical property of a commercially available CF. Second, we formulate the joint sensing device’s geometric synthesis procedure as a constrained revolute joint system, where the CF is designed and introduced as an RPR chain to obtain an ISC linkage. Lastly, we designed our wearable sensing device and validated against an ISC linkage fixture representing an elbow joint and an actual healthy human subject’s left elbow. The ISC linkage fixture experimental setup shows that our designed joint sensing device can track the elbow extension motion of 140 deg with a maximum error of 7.66%. The results from our human subject’s left elbow show that it can track the elbow flexion–extension at various angular motion, with error ranges between 8.24 deg and 12.86 deg, and have provided us with an acceptable average Spearman’s coefficient values rs at 0.95.