Mitte died in Brownsvillle, Texas, on Monday. His publicist confirmed that he took his own life.
Cranston praised Mitte's advocacy for performers with disabilities, and noted that he never allowed his cerebral palsy to "be an excuse for anything".
"All that promise and potential now is such a loss," wrote the actor, adding that he "thoroughly enjoyed being with him throughout the years on the series, and after the spotlight on Breaking Bad dimmed".
He said "it is especially devastating to hear that he took his own life."