Clinton won nearly 79 percent of the vote against 21 percent for Bernie Sanders, with nearly all votes counted, according to US networks.
It marked a deflating finish for the Vermont senator, who captivated liberals and independents with a grassroots campaign that mounted a surprisingly strong challenge to Clinton.
The attention of the candidates -- and the nation -- however has shifted to the aftermath of the Orlando shooting, the deadliest terror attack on US soil since September 11, 2001.
Clinton and Trump traded verbal blows and presented dramatically different approaches for fighting terrorism following the massacre at the Florida gay nightclub.
Trump, the presumptive Republican nominee, on Monday said that if elected he would "suspend" immigration from areas with a "proven history of terrorism."
He also suggested that US Muslims were complicit in domestic attacks because they failed to "turn in the people who they know are bad."
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In contrast, former secretary of state Clinton called on Americans to "stand together" to defeat terrorism.
But after Trump suggested in a TV interview that Obama sympathized with terrorists, Clinton on Tuesday slammed her rival's approach "dangerous" and "un-American."
"Even in a time of divided politics, this is way beyond anything that should be said by someone running for president of the United States," she told supporters in Pittsburgh.
"What Donald Trump is saying is shameful ... more evidence that he is temperamentally unfit and totally unqualified to be commander in chief," she said.
Healing party divisions
Washington's primary was an afterthought as Clinton reached number of delegates needed to lock up the nomination last week.
Sanders however refused to concede, though he steadily softened his tone in recent days.
Sanders and Clinton met at a Washington hotel "and had a positive discussion about their primary campaign, about unifying the party and about the dangerous threat that Donald Trump poses to our nation," a Clinton campaign official said after the meeting.
Sanders spokesman Michael Briggs said the two "had a positive discussion about how best to bring more people into the political process" and about the threat posed by Trump.
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Clinton and Sanders also discussed what would be on the Democratic Party platform ahead of the national convention next month in Philadelphia.
Sanders told reporters ahead of the meeting that he wanted to see "the most progressive platform ever passed" at a convention, one which "makes it crystal clear that the Democratic Party is in fact on the side of working people."
Sanders met with President Barack Obama last week and emerged from the White House declaring his intent to work with Clinton to defeat Trump in November.
Obama endorsed Clinton later that day.
Republicans in Washington held their party convention in March, with Senator Marco Rubio emerging as the top vote-getter.
Tuesday's vote wraps up a spectacular primary season that saw conservatives flock to Trump, a celebrity billionaire and political novice, and liberals propel Sanders, a self-described democratic socialist, into the national spotlight.
Sanders tapped into a deep well of anger among voters disillusioned with the current political system and eager to see action on issues such as reducing income inequality and campaign finance reform.
Clinton ultimately prevailed, becoming the first female presumptive presidential nominee of any major US political party.
Congresswoman and Democratic National Committee chair Debbie Wasserman Schultz congratulated both candidates for having "energized voters across the country."
"Now that our 2016 primaries are officially at their end, Democrats are ready to unify and take on both Trump and the Republican Party that he represents," Wasserman Schultz said in a statement.
Sanders is scheduled to address supporters live via webcast Thursday.
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