Following Europe's demolition of the United States in France past year, Harrington had been viewed as the only candidate to preside over a defence.
The appointment of the three-time major champion, 47, was announced in a news conference at Wentworth on Tuesday.
The three-time major victor succeeds Dane Thomas Bjorn and becomes just the second golfer from the Republic of Ireland to take the reins for the tournament after Paul McGinley, who was victorious in 2014.
The Irishman will become the third captain from the Emerald Isle in four Ryder Cups.
Harrington was chosen as Europe's 2020 Ryder Cup captain by a five-man selection panel comprising the three most recent European Ryder Cup captains - Bjørn, Clarke and McGinley - as well as the Chief Executive of the European Tour, Keith Pelley, and European Tour Tournament Committee representative Edoardo Molinari.
"Becoming captain is not something I went into lightly", Harrington said after he was appointed.
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"You could look at this as being a natural progression from player to vice-captain to captain but it's not something I take on without a certain amount of trepidation". "I don't think I ever thought I wasn't going to do it, but I wanted to make sure I was doing it for the right reasons".
"I know we are going to a new venue, it's an away match".
Several key Europe players, including Northern Ireland's Rory McIlroy and England's Justin Rose, had given Harrington public backing to lead the team in their title defence.
"The reminder we have every day is the Harrington Room where we have all his trophies from his amateur days right through to the Walker Cup, Ryder Cup, then the Majors and competitions around the world", Mr King said. Speaking about his experience at Le Golf National, Harrington said: "You are always learning and it was a nice balanced vice-captaincy with different personalities; some good emotions, some good thinkers".
Harrington, 47, was a Ryder Cup victor in four of his six appearances as a player while he was a vice-captain in 2014 and 2016, as well as 2018.
"It does make my job easier that I feel I have the confidence of the players", Harrington added. We've had some great captains over the years and I've learned from all of them. "I am very much looking forward to taking on this role". Thankfully we are going to Whistling Straits which is just a strong golf course. I know they want continuity. I know it's an uphill struggle to beat the U.S.in the USA but these are good golf courses.
While the US captain for 2020 has not yet been announced, 2017 Presidents Cup captain and Wisconsin native Steve Stricker is perceived as the favorite to lead the American squad in his home state.