In the three months to 30 September, the late-deals specialist posted a pre-tax surplus of £302,000, compared with a pre-tax loss of £11 million in the equivalent period last year.
Customer conversion rose to 22.6 per cent, up from 13.9 per cent, while customer acquisition costs fell to £10 from £38, and from £25 to £5 in the UK.
However, the pre-tax profit only comes after discounting a one-off exceptional charge of £3.1m for redundancy and surplus property costs following the acquisition of European firm Travelprice.com in July.
Martha Lane Fox, lastminute.com's group managing director, said: "It's a first milestone, and has come a year ahead of our original business plan. This time next year is when a full-year pre-tax profit is predicted."
She added that lastminute.com's UK operation had grown 100 per cent organically year on year, separate to the group's acquisitions of The Destination Group, Travelselect.com and Travelprice.com.
The company is in a period of consultancy over redundancies while Travelprice.com's operations are integrated. The process will save the group £6m; Lane Fox said that about 30 per cent of staff from the combined business would lose their jobs.