We explore the conductive mechanism of yarns made from metallic fibers and/or traditional textile fibers. It has been proposed for the first time, to our knowledge, that probe span length plays a great role in the conductivity of metallic fiber-based yarns, which is determined by the probability and number of conductive fibers appearing on a cross section and their connecting on two neighboring sections in a yarn’s longitudinal direction. The results demonstrate that yarn conductivity is negatively influenced to a large extent by its length when metallic fibers are blended with other nonconductive materials, which is beyond the scope of conductivity theory for metal conductors. In addition, wicking and wetting performances, which interfere with fiber distribution and conductive paths between fibers, have been shown to have a negative influence on the conductivity of metallic fiber-based yarns with various structures and composed of different fiber materials. Such dependence of the conductivity on the probe span length, as well as on the moisture from air and human body, should get attention during investigation of the conductivity of metallic fiber-based composites in use, especially in cases in which conductive yarns are fabricated into flexible circuit boards, antennas, textile electrodes, and sensors.