But the two had a major falling out after the 2016 election, as investigators began looking into several of Mr Trump's aides.
In 2018, Cohen was sentenced to three years in prison and a fine after pleading guilty to charges of fraud and campaign finance violations.
Now out of prison, Cohen has become a high-profile critic of Mr Trump, and a frequent guest on news programmes.
He has written a book and hosts a podcast, both of which Mr Trump cites in the lawsuit.