Let's call the measure of the angle x.
By definition, the supplementary angle of x measures 180 - x degrees.
We're told that the angle measures 120 degrees less than its supplementary angle, so we can set up an equation:
x = (180 - x) - 120
Simplifying this equation, we get:
x = 60
Therefore, the angle measures 60 degrees, and its supplementary angle measures 180 - 60 = 120 degrees.