Mr Desmond, chairman of Northern & Shell, described the deal as a "transformational transaction".
"This deal is a really exciting moment in Trinity Mirror's history, combining some of the most iconic titles in the United Kingdom media industry", said Trinity Mirror chief executive Simon Fox.
Simon Fox, the chief executive of Trinity Mirror, told Radio's 4 Today programme the deal is "a very wise investment" despite news groups facing intense pressure on both print and digital advertising because of the Facebook-Google duopoly and changing consumer habits.
Trinity said it expects to generate GBP20 million per year in synergies from the deal by 2020, with a "significant amount" of these cost savings to be achieved by 2019.
"Rather than sending many reporters to cover the same football games, we can cover more games, more sports and offer deeper and wider content", said Fox, on how the different titles will pool resources and save costs.
While other media owners have sold out of newspapers in recent years, such as Pearson selling the Financial Times and Rupert Murdoch putting his newspapers in a separate company, Trinity Mirror has doubled down on the industry.
Desmond, who bought the Express newspaper titles for £125m in 2000 from UBM's former guise of United News & Media, added: "The Express newspapers and our celebrity magazine titles have been a key part of the Northern & Shell portfolio for many years, and I am immensely proud of building them into one of the largest newspaper and magazine groups in the UK". The Express's front-page formula of health, weather and migrants is carefully aimed at its loyal but shrinking band of older, conservative readers.
Then, in January 2017, Trinity said it was in talks to take a minority stake in a new company containing some assets of Northern & Shell. The Express was one of the few papers to oppose joining the Common Market in the early 1970s, although it did eventually back "yes" in the 1975 referendum. New! and Star- since autumn. According to commentators, the move will create a company better able to cope with readers and advertisers moving online.
Trinity Mirror, also the UK's largest regional newspaper owner, will look to make efficiency savings by merging roles across the titles.
He is reported to be contemplating whether to bid for the licence to run the official National Lottery.
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