"Great news to wake up to", said Donald, who played on three Ryder Cup teams with Harrington in 2004, 2006 and 2010.
"It's not something I went into lightly", he said.
"I know they want continuity", Harrington said, referring to Europe's top players, "and it is a very important thing".
"I will definitely take bits from all of them".
Pádraig Harrington celebrates with future Ryder Cup captains Darren Clarke and Paul McGinley at the 2002 event.
"I'm excited with the whole idea, I hope I can add something to it", added Harrington.
"He'll be very diligent, there's no doubt about that, very confident and hopefully continue this great run of success we're having in the European team". I suppose I've learnt from all of them, I've probably learnt a lot more as vice captain than you do as a player, it's incredible the difference. He's got a lot of things in his favour. It is really tough to win a Ryder Cup in America, but there is nobody more suited to taking on that challenge.
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 played in six Ryder Cups (four wins) and was vice captain at the last three (Gleneagles, Hazeltine and Paris).
However, the 47-year-old is well aware of the damage a Ryder Cup defeat can do to a captain's reputation having played under six-time major victor Sir Nick Faldo in Europe's defeat at Valhalla in 2008.
Nearly from the time the last putt dropped at the Ryder Cup in France, Harrington was deemed the likely pick for 2020.
"I'm the right man for the job at this time", he said.
"It really came down to whether I wanted to be in the hat and put what is a successful career, put it on the line".
Harrington is known to have been a popular choice among the team that stormed to victory in Paris a year ago. A Ryder Cup captain's job is, essentially, to rewire that mindset.
He said: "Steve's a solid bloke". "Good guy. Tough. Not a man to underestimate". I'm sure there's going to be a certain edge to it but look it's a big strong golf course and we should all know what we are getting.
While latterly he has brought his extensive experience to the role of vice-captain in the past three editions of the biennial contest. When you're a player, you are exclusively concerned with your ability to perform on the biggest stage of all and trying to get your game in optimum shape. He's as straight as they come.
"Padraig is playing the best golf of my team out there", Montgomerie told reporters during the first official practice day at Celtic Manor Resort. "But there's no messing from his side".