Coach Steve Kerr described the triumph as "the hardest so far" but his Warriors side routed the Cavs 108-84 to prevail in the fourth consecutive finals meeting between the teams.
Stephen Curry scored 37 points for the Warriors, while LeBron James claimed 23 in what could be his final Cavs game.
James is a free agent next month.
The 33-year-old American, playing in his eighth consecutive finals, said he had "no idea at this point" about his future and would consider the decision with his family.
The win added to the Golden State Warriors' victories in 2015 and 2017 and was the first finals clean sweep since the Cavs were demolished by San Antonio Spurs in 2007.
It was the second time the Warriors have secured the championship in Cleveland during Kerr's four-year tenure.
"I remember sitting in this room three years ago and it seemed like a dream. This feels like reality," Kerr said. "I hope that doesn't sound arrogant. It's just the talent we have and the experience we've gained."
A James slam dunk had Cleveland ahead 39-38 halfway through the second quarter, but Golden State responded to lead 61-52 at half-time and eased to victory from there.