The Northern Irishman, who led the tournament after each round, carded a final round of 71 at Hoylake to finish 17 under and secure his third career major.
He joins Jack Nicklaus and Tiger Woods in winning three majors by the age of 25 and McIlroy confessed that the achievement would take some while to sink in.
Having left Sergio Garcia and Rickie Fowler in tied second, McIlroy highlighted his birdie at the 16th hole as the turning point.
"It feels incredible. Today wasn't easy, a few guys were making runs at me, I needed to stay focused and concentrate," he told the BBC.
"To win three legs of the four majors at 25 is a pretty good achievement, it is not going to sink in for a while.
"The lead never got less than two, I always felt I had that little cushion.
"Coming into the back nine I know I had holes where I could make birdie. Sixteen was the hole that settled the championship I think."
The victory marks a huge improvement on McIlroy's effort at the 2013 Open, when he missed the cut after carding rounds of 79 and 75 at Muirfield.