The man who led the club to the 1997 FA Trophy triumph at Wembley has taken over from Graeme Clark, who resigned last week.
Dunn left his position as manager of Conference North Blyth Spartans at the end of the season and could not resist the lure of his former club. He told the Whitby Gazette it was not a difficult decision to come back.
"I enjoyed my time at Whitby hugely and we had some success and I'm hoping I can bring some of that success back," he said.
"You never say never in football and it didn't take long for me to decide to come back. Chairman Graham Manser told me he wanted to push on this season and make a run at promotion and that's what I wanted to hear."
Dunn is widely regarded as Whitby's most successful manager and got a first-hand view of the Blues last season when Blyth clashed with Town in an FA Cup qualifier, with Spartans squeezing through in a replay.
"To be honest I thought we (Blyth] were going to get knocked out the cup," he said.
"Whitby were unlucky in both games, I thought Ashley Lyth was hard done to get a red card in the first minute at Blyth and I thought there might have been a foul on Whitby's keeper for our second at the Turnbull," he said. "They looked a very good team and matched us in every department."
Blyth were eventually knocked out by Premier League Blackburn Rovers – and Whitby fans will be hoping he can repeat the cup run of his first spell which saw the Blues take Plymouth Argyle to a replay in front of Match of the Day cameras.
Harry is renowned for his contacts in the game and has admitted he has already sounded out a number of players about a move to the Turnbull.
"There are going to be some good players out there this summer and I've already spoken to eight or nine. That doesn't mean I am looking for that many but if you speak to eight or nine then you might get in three, four or five.
"There are no particular positions I am looking at as I believe you have to have the best players.
"If there is a player out there who is better than the player you have got, you should get them in."
He also paid tribute to Whitby's faithful fans who set up a Facebook group calling for his return to the hot seat.
"When you have 300 passionate fans behind you every week it really makes the difference," he said.
"The ground and the pitch are looking great and I can't wait for that first game."

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