Johnson has steered the distribution network operator (DNO) through two price reviews during his seven years at the company.
He said: “The time is right to move on in the knowledge that I leave a network that is more reliable, more efficient and delivers better customer service to the people of the North West.”
Johnson said he was leaving ENW “to take on new challenges”.
He said he was “immensely proud” of ENW being recognised as the most efficient DNO in the most recent price review, and also the company’s record of innovation.
In June the company launched a three-year project with Schneider Electric looking at how a smarter grid can help networks automatically fix issues.
Johnson joined ENW in 2008 from his role as Morrison Utilities Services’ managing director. He remains non-executive director at South West Water.
ENW said a new chief executive will be announced “in due course”.