According to the Electoral Commission's latest figures, Reform UK received donations totalling more than £10.2m between July and September.
This was more than the Conservatives, who received £4.6m in donations, followed by Labour on £2.1m and the Liberal Democrats on £1m.
Aside from Mr Harborne's donation, the next biggest during the three-month period was £1m given by games programmer and tech entrepreneur Jeremy Elliott San to the Conservatives.
Labour's biggest donor was the Unite union with £362,625, while the Liberal Democrats received its largest sum - £50,000 - from Neale Powell-Cook, who owns a food supply company.
It is the first quarter since last year's general election that Reform has received more in donations than the Conservatives.
A Reform spokesman said the figures "show the incredible progress" the party is making.
"This is further evidence that we have all the momentum in British politics," he added.
However, overall the Tories have still received more donations since July 2024 - around £14.4m compared with £13.5m for Reform.
Conservative leader Kemi Badenoch said the Tories had been "out-raising" other parties throughout this year, while Mr Harborne's donation to Reform was a "one-off".
"If you look at the breadth and depth of our donors, and the trend, I'm really, really pleased with the announcement today that shows that we've had our best performing quarter since I became leader," she told broadcasters.
Liberal Democrat spokesperson Lisa Smart said the latest figures showed "some eye-watering donations".
"Our elections are not being fought on an even footing if they can be bought by a small handful of individuals," she added.
Political parties are required to report all donations above £11,180 to the Electoral Commission.
Opposition parties also receive public funds to support their parliamentary duties, with allocations based on their performance at the previous election.
Between July and September, Reform received £227,835 in public funds - far less than the £2.3m received by the Conservatives and the £1m for the Liberal Democrats.