US district judge Matthew Brann in Williamsport turned down the request for an injunction by president Donald Trump's campaign, spoiling the incumbent's hopes of overturning the results of the presidential contest.
In his ruling, Mr Brann said the Trump campaign presented "strained legal arguments without merit and speculative accusations... unsupported by evidence".
"In the United States of America, this cannot justify the disenfranchisement of a single voter, let alone all the voters of its sixth most populated state.
"Our people, laws, and institutions demand more."
Former federal prosecutor and New York City mayor Rudy Giuliani returned to court for the first time in decades to represent the Trump campaign on Tuesday.
He repeatedly contended in court that it was illegal for counties to help people vote.
Opposing lawyer Mark Aronchick suggested Mr Giuliani must not know the Pennsylvania election code.
Mr Trump had argued that the US Constitution's guarantee of equal protection under the law was violated when Pennsylvania counties took different approaches to notifying voters before the election about technical problems with their submitted mail-in ballots.
Pennsylvania secretary of state Kathy Boockvar and the seven Biden-majority counties that the campaign sued had argued Mr Trump had previously raised similar claims and lost.
They told Mr Brann the remedy the Trump campaign sought, to throw out millions of votes over alleged isolated issues, was far too extreme, particularly after most of them have been tallied.
Pennsylvania attorney general Josh Shapiro, a Democrat, tweeted shortly after Mr Brann's ruling, saying "another one bites the dust".
"These claims were meritless from the start and for an audience of one," he said in a statement. "The will of the people will prevail. These baseless lawsuits need to end."
The state's 20 electoral votes would not have been enough on their own to hand Mr Trump a second term.
Counties must certify their results to Ms Boockvar by Monday, after which she will make her own certification.
Democratic governor Tom Wolf will notify the winning candidate's electors they should appear to vote in the Capitol on 14 December.