Sales in September of the compact, which costs 50p, were up 4% to an average 290,000 per day.
However, revenue for the group, which also publishes The Scotsman, fell 3% between July and October when compared with the same period last year.
The figures came on top of a 9% fall in revenue for the first half of 2016.
The growth of the i resulted in the title taking a 20% share of the so-called "quality segment" of Britain's newspaper market.
Without the i, the figures fell more sharply, with revenues down 16% in the past three months.
Advertising revenue without the new title was also down 12%, while circulation revenue dropped by 10%.
Digital advertising, seen as a way of replacing declining revenue from paper sales, fell during the summer-autumn period by 3%.
Despite the figures, Johnston Press saw its share price rise slightly to 14p.
It has fallen from 66p a year ago, and fell below 9p before a surge in September, when an activist investor became involved in moves to change the publisher's strategy.
Last month, it announced a deal with creditors to change the conditions on some of its borrowing, giving it more time to raise finance by selling assets.
That is expected to include newspaper titles.
The growth of the i title has helped drive a rise in sales for Edinburgh-based newspaper publisher Johnston Press.