Marroni, formerly head of global media relations at Freshfields, will replace Bruce Wraight, who has been at the company for three-and-a-half years and told PRWeek he was leaving shortly to pursue other projects.
Marroni, who joined Freshfields in 2008 and left last month, will start on 28 April and will head a team of nine.
Linklaters has 29 offices around the world with an array of clients including Barclays, Prudential, RSA Insurance Group and Vedanta Resources.
Freshfields is also an international law firm with more than 2,400 lawyers in 28 offices and clients including Daimler, Rolls-Royce and BAE Systems. It has advised its oldest client, the Bank of England, for more than 250 years.
In the FTSE 100 list published in The Lawyer, Linklaters clients grew from 24 in the first quarter of 2013 to 28, while Freshfields was static, representing 24 FTSE 100 clients.
Marroni has not yet been replaced. PR managers Alexa Highfield and Nick Parker continue to support the firm.
Both Freshfields and Linklaters declined to comment on the appointment.