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The 21-year-old, from Meon Valley Golf Club, followed in the footsteps of fellow Hampshire golfer Scott Gregory in 2016 by winning the trophy.
The victory guarantees Ellis will play at Royal Birkdale next month as well as the Masters and US Open next season.
"The magnitude of what I did is starting to sink in," Ellis said.
Ellis came from four holes down with five remaining in the 36-hole final to beat Australia's Dylan Perry at Royal St George's in Kent on Saturday.
Victory was only secured after two play-off holes for the man who became the youngest to win the English Amateur Championship, aged just 16, in 2012.
"I still feel a little numb," he told BBC South Today. "The task was getting a lot tougher to come away with the trophy and I knew I just had to keep myself going.
"It all came out when the winning putt was sunk. It was years of hard work and a lot of disappointment matched with the excitement of holing that putt."
Ellis has just completed his third year on a scholarship at Florida State University, where he hopes the experience will stand him in good stead for an appearance at Augusta next season.
He revealed US Open champion Brooks Koepka was among those to text their congratulations after Saturday's win.
"Justin Rose and Scott Gregory also sent some nice messages," Ellis said. "It's been really nice to receive those and I'm really excited for the opportunities ahead."
The Open begins at Royal Birkdale on 20 July.
Harry Ellis admits he still "feels numb and in shock" after securing his spot at The Open in July by winning the British Amateur Championship.