Byrne, who has spent more than two decades at Weber Shandwick, announced in May that he would leave the agency next year. He has now confirmed the March date.
Weber Shandwick is expected to announce Byrne's successor later this year.
Byrne told PRWeek he was stepping away from PR to complete a Masters degree in creative writing, saying: "I have no plans to take up further roles in PR."
He added that after 35 years in comms - including 23 years with Weber - it was "time for something new". Byrne said he wanted to "study, write and be a full-time dad to my three young boys".
Byrne joined Weber in 1995 after eight years as the chief press officer for the UK Labour Party, and was promoted to lead the London public affairs practice in 1997. He then became UK CEO in 2000 and added the EMEA region to his duties in 2008.
Weber Shandwick recorded 11 per cent growth in the UK last year, according to PRWeek's 2017 Top 150 Consultancies rankings table, meaning the firm consolidated its position as the third biggest agency in the UK.