We present a mass-luminosity relation (MLR) for red dwarfs spanning a range of masses from 0.62 M-circle dot to the end of the stellar main sequence at 0.08 M-circle dot. The relation is based on 47 stars for which dynamical masses have been determined, primarily using astrometric data from Fine Guidance Sensors (FGS) 3 and 1r, white-light interferometers on the Hubble Space Telescope (HST), and radial velocity data from McDonald Observatory. For our HST/FGS sample of 15 binaries, component mass errors range from 0.4% to 4.0% with a median error of 1.8%. With these and masses from other sources, we construct a V-band MLR for the lower main sequence with 47 stars. and a K-band MLR with 45 stars with fit residuals half of those of the V. band. We use GJ 831. AB as an example, obtaining an absolute trigonometric parallax,pi(abs) = 125.3 +/- 0.3 mas, with orbital elements yielding. M-A=0.270 +/- 0.004 M-circle dot and M-B = 0.145 +/- 0.002 M-circle dot. The mass precision rivals that derived for eclipsing binaries. A remaining major task is the interpretation of the intrinsic cosmic scatter in the observed MLR for low-mass stars in terms of physical effects. In the meantime, useful mass values can be estimated from the MLR for the ubiquitous red dwarfs that account for 75% of all stars, with applications ranging from the characterization of exoplanet host stars to the contribution of red dwarfs to the mass of the universe.