The 35-year-old had been billed as the man to at least take Joshua beyond three rounds but was saved by the bell in the first.
An Ali shuffle from the American had drawn laughter from the crowd at the O2 Arena but Johnson looked shocked by Joshua???s power when a heavy right forced him to go down on one knee, clinging onto the ropes for support.
There was then serious concern for Johnson???s condition, when a sustained assault of rights at the end of the first sent the veteran through the ropes.
To Johnson???s enormous credit, he came out for the second round but was immediately under pressure, with Joshua teeing up his bludgeoning rights with short left upper cuts.
Johnson was unable to offer anything in return and the referee had no choice but to leap to his defence.
The win improves Joshua to 13-0 with 13 knockouts ??? all within three rounds ??? and, after seeing his charge despatch former world title challenger Johnson with such ease, promoter Eddie Hearn promised a big-name opponent for Joshua on September 12 at the O2 Arena.??