  The James Webb Space Telescope (JWST), currently scheduled to launch in 2021,
will dramatically advance our understanding of exoplanetary systems with its
ability to directly image and characterise planetary-mass companions at wide
separations through coronagraphy. Using state-of-the-art simulations of JWST
performance, in combination with the latest evolutionary models, we present the
most sophisticated simulated mass sensitivity limits of JWST coronagraphy to
date. In particular, we focus our efforts towards observations of members
within the nearby young moving groups $\beta$ Pictoris and TW Hya. These limits
indicate that whilst JWST will provide little improvement towards imaging
exoplanets at short separations, at wide separations the increase in
sensitivity is dramatic. We predict JWST will be capable of imaging sub-Jupiter
mass objects beyond ~30 au, sub-Saturn mass objects beyond ~50 au, and that
beyond ~100 au, JWST will be capable of directly imaging companions as small as
0.1 $M_\textrm{J}$ - at least an order of magnitude improvement over the
leading ground-based instruments. Probing this unexplored parameter space will
be of immediate value to modelling efforts focused on planetary formation and
population synthesis. JWST will also serve as an excellent complement to ground
based observatories through its unique ability to characterise previously
detected companions across the near- to mid-infrared for the first time.
