We use the positions and velocities of satellites of our galaxy and of other spiral galaxies to determine the radial mass profile of dark matter halos. We combine our measurement of the velocities of five remote Galactic satellites with published observations of the other Galactic satellites to obtain a complete sample of test particles. We then apply statistical techniques and timing arguments to deduce that the mass of the Galaxy is > 1.3 X 1012 solar mass for standard assumptions and that the halo extends beyond 100 kpc Galactocentric distance. We confirm our results by examining the dynamics of other Local Group galaxies. Subsequently, we expand our study to include nearby (1000 km s-1 200 kpc) and massive (> 1012 solar mass), M/L > 80) dark matter halos around isolated unbarred late-type spiral galaxies.