Mail Online has long been the most popular newspaper site in the UK and, according to comScore figures, overtook the New York Times to become the leading online English-language newspaper at the start of the year.
The source said: "We always knew we would achieve a monthly operating profit this summer, with total revenues up more than 75% year on year, but to do it in June was very welcome."
The site’s move into the black comes despite the publisher investing in expanding its US operation and providing specific content for India.
The site's unparallelled growth in vistors over the past five years has been achieved by fewer than 30 people in the UK, a team of 20 in New York, and 10 in Los Angeles.
Last year, Mail Online’s revenues were £16m, and in May, Mail Online publisher, Martin Clarke, forecast digital revenues will hit £25m this year, £45m next year, and £100m by 2017.
Such growth is in stark contrast to the fortunes of DMGT’s print operations, which posted a 26% fall in operating profits at its national newspaper division in the six months to 1 April.
Print losses overshadowed the digital performance and Associated, the national newspaper division that is home to the Daily Mail and Mail on Sunday, reported a 26% fall in operating profit to £34m and a 1% decline in revenues to £435m.
Mail Online attracted more than 5.6 million daily unique browsers in May, the last ABCs, and almost 92 million monthly uniques."