Timothy Mellon, a reclusive heir to a Gilded Age fortune, donated $50 million to a super PAC supporting Donald J. Trump the day after the former president was convicted of 34 felonies, according to new federal filings, an enormous gift that is among the largest single disclosed contributions ever.

The donation’s impact on the 2024 race is expected to be felt almost immediately. Within days of the contribution, the pro-Trump super PAC, Make America Great Again Inc., said in a memo that it would begin reserving $100 million in advertising through Labor Day.

The group had only $34.5 million on hand at the end of April, and Mr. Mellon’s contribution accounted for much of the nearly $70 million that the super PAC raised in May. On Wednesday and Thursday, the super PAC began reserving $30 million in ads to air in Georgia and Pennsylvania around the Fourth of July holiday.

Mr. Mellon is now the first donor to give $100 million in disclosed federal contributions in this year’s election. He was already the single largest contributor to super PACs supporting both Mr. Trump and Robert F. Kennedy Jr., who is running as an independent. Mr. Mellon has previously given $25 million to both.

Democrats have sought to portray Mr. Kennedy as a spoiler supported by Republicans, in part by emphasizing Mr. Mellon’s dual contributions and seemingly split loyalties. The pro-Kennedy super PAC has distributed quotations from the hard-to-reach Mr. Mellon, and for a blurb that appears on the cover of Mr. Mellon’s upcoming book, Mr. Kennedy called the billionaire a “maverick entrepreneur.”

It is not clear what Mr. Mellon’s mega-donation means for his support of Mr. Kennedy going forward. He has so far toggled between giving to support both candidates. His most recent donation to Mr. Kennedy’s super PAC was a $5 million contribution in April.

But Mr. Mellon’s $50 million gift will significantly help pro-Trump forces narrow the financial advantage that President Biden and his allies have enjoyed so far. Miriam Adelson, the casino billionaire and widow of Sheldon G. Adelson, who died in 2021, has also made plans to fund a pro-Trump super PAC with at least as much money as the $90 million that her family gave in the 2020 campaign, although much of the cash has yet to arrive.

Richard and Elizabeth Uihlein, the Illinois couple who are among the G.O.P.’s largest donors, each gave $5 million to the Trump super PAC in May. The billionaire energy executive Kelcy Warren also gave $5 million.

But outside groups supporting Mr. Biden have already announced more than $1 billion in planned spending, anchored by a reserved $250 million in advertising from the leading pro-Biden super PAC, Future Forward.

Individual donations as large as $50 million are rare in American campaigns. Other gifts of a similar size have come from candidates who self-funded their campaigns, from couples who technically split their mammoth contributions or from donors who have paid in installments over time.

Until now, Make America Great Again Inc., which serves as the leading pro-Trump super PAC, has had only modest fund-raising success, relying largely on Republican donors who have personal connections to the former president.

In the first few months of 2024, the group raised between $7.4 million and $14.4 million a month. MAGA Inc. was originally seeded with $60 million by Mr. Trump’s political action committee — which is prohibited from spending to support his candidacy — before he declared his run for president. But in a highly unusual transaction, Mr. Trump later asked for a refund of the $60 million he had given months earlier, so MAGA Inc. has now returned that amount to the PAC, Save America, which is helping pay his legal bills.

Mr. Mellon, who had previously put $25 million into the group over the last 12 months, now accounts for nearly half of what the group has raised in total.

Mr. Mellon has long avoided the publicity that typically surrounds a donor this significant. After bursting onto the Republican fund-raising scene at the dawn of the Trump administration, he quickly developed a reputation as an unusual, quirky figure.

Despite his famous last name — he is the grandson of former Treasury Secretary Andrew Mellon and a member of the wealthy Mellon family — Republican fund-raisers had largely not heard of him before he made a $10 million donation to a G.O.P. super PAC in mid-2018. That gift was the first of nine eight-figure checks that he would cut to major Republican groups.

He would go on to hire political counsel to guide him in Washington, although he lives primarily in Wyoming these days. Few recipients of his money have even met him.

The $50 million check to support Mr. Trump is matched only by a different donation Mr. Mellon made on behalf of another tough-on-immigration political project: the private construction of a border wall in Texas. In August 2021, Mr. Mellon donated $53 million worth of stock to help pay for the wall, a priority of Gov. Greg Abbott of Texas.

Mr. Mellon, who did not respond to requests for comment on Thursday, appears to be growing more comfortable with the scrutiny of his influence. Next month, he is slated to publish a book, “panam.captain,” about his work turning around Pan Am Systems, a collection of companies that includes rail, aviation and marketing firms.

Mr. Mellon originally self-published an autobiography, but it was taken off-line in 2016 after some incendiary passages became public, including a line that Black people were “even more belligerent” after social programs were expanded in the 1960s and ’70s.

Mr. Mellon also wrote that social safety net programs amounted to “slavery redux.”

“For delivering their votes in the Federal Elections, they are awarded with yet more and more freebies: food stamps, cellphones, WIC payments, Obamacare, and on, and on,” Mr. Mellon wrote, according to The Washington Post.

The new book, “panam.captain,” will be released by Skyhorse Publishing. Its president is Tony Lyons, who co-founded the pro-Kennedy super PAC, American Values 2024.

In a rare interview with Bloomberg in 2020, Mr. Mellon praised what he saw as Mr. Trump’s follow-through: “He’s done the things he promised to, or tried to do the things he’s promised to,” he said.