Sounds like the unprecedented override was the beginning of the end for him.
He was impeached for firing Secretary of War Stanton in violation of the Tenure of Office Act which Congress had enacted to keep President Johnson from firing Lincoln's cabinet...a law that was likely very unconstitutional.
Those were insane times. You had military generals deposing elected state governors and replacing them with their own appointees...Congress admitting Nebraska as a state over the president's veto so that Republicans could get two more votes and another state ratifying the 14th Amendment...Congress passing unconstitutional laws restricting the powers of the president, and then impeaching him for violating those statutes. Then again, we had just had the Civil War...things were bound to be crazy, as every act of government was unprecedented.
Johnson was the only president to be elected to the Senate after being president.
Johnson spoke out against the "despotism" of Grant, who used the military to support Louisiana's Reconstruction Government.