Kasowitz has served as a representative for Trump "on a wide range of litigation matters for over 15 years," according to his law firm's website, both in Trump's businesses and during the presidential campaign.
Kasowitz co-founded the New York City law firm now known as Kasowitz Benson Torres in 1993, after earning a reputation as a hard-hitting litigator representing big tobacco. He graduated from Yale University and Cornell Law School.
His firm, formerly Kasowitz Benson Torres Friedman until partner David Friedman became Trump's pick for U.S. ambassador to Israel, represents corporations and real-estate actions and touts representing the businessman-turned-president across a number of dealings.
Kasowitz Benson Torres was retained to restructure bondholder debt on Trump's casinos in 2001, sue investors over the sale of a real estate development in 2005 and pursue a defamation case over claims made in a biography of the future president in 2006, among other matters.
More recently, Kasowitz was hired by the Trump campaign to push back against sexual harassment accusations made in an October New York Times article.
"Your article is reckless, defamatory and constitutes libel per se," wrote Kasowitz in a letter to Times Executive Editor Dean Baquet. "It is apparent from, among other things, the timing of the article, that it is nothing more than a politically motivated effort to defeat Mr. Trump’s candidacy."
Despite the attorney's claim that Trump would "pursue all available actions and remedies" if the story was not retracted with an apology, the newspaper did not do so. Ultimately, Trump did not pursue legal action.
Kasowitz was successful during the campaign in keeping the record of Trump's divorce from his first wife Ivana sealed after the Times and Gannett attempted to have it released.
Trump was effusive in his praise of Kasowitz's firm in a 2004 story in American Lawyer which quoted the businessman as saying, "[they're] not good lawyers, they’re phenomenal lawyers.”
"They’re highly talented with great insight into the future," he added.
In addition to his work for the president, Kasowitz has developed a reputation for his representation of high-profile financial services firms in suits involving various issues, including bankruptcies, fraud, buyouts and restructuring.
Among Kasowitz's colleagues at his law firm is former Connecticut Sen. Joe Lieberman, whom Trump labelled as his top choice for the vacant position of FBI director two weeks ago, after the firing of James Comey.
Lieberman withdrew from consideration last week over suggestions that his appointment to lead the bureau would be improper given his position at Kasowitz Benson Torres.
"With your selection of Marc Kasowitz to represent you in the various investigations that have begun, I do believe it would be best to avoid any appearance of a conflict of interest, given my role as senior counsel in the law firm of which Marc is the senior partner," wrote Lieberman in a letter informing Trump of his decision.