In this paper, we propose a new method to extract the free carrier mobility in junctionless (JL) double-gate FETs biased in accumulation. We show that, in addition to assuming a well-defined field-dependent mobility law, using the well-known Y-function is not accurate enough in JL FETs. In addition, the new methodology is also interesting for inversion-mode FETs. This approach has been assessed with technology computer-aided design simulations, and confirms its robustness both for JL and inversion-mode devices.