Reilly is a former regional journalist, including for the Dover Express, but has spent the bulk of his career since 2008 working for the Lib Dems' comms operation.
He began working for the Lib Dems as a press officer at the party’s HQ in London's Cowley Street but rose to become deputy head and then head of the press office during the coalition.
Reilly spent three months working on former Met Police chief Brian Paddick's 2012 unsuccessful bid to be London Mayor, and his ascent continued in 2014 when he took the role of head of government comms to the Deputy Prime Minister at Number 10 and then became Nick Clegg’s chief speechwriter.
As chief speechwriter, Reilly contributed to Clegg’s 2015 pre-election conference speech – described by veteran political journalist Michael Crick as "one of the best speeches I've heard from a party leader in years" – as well as his resignation speech, following which 10,000 new members joined the Lib Dems.
Reilly continued to work with Clegg as his spokesman after the election, as well as helping the former Lib Dem leader to write a book.
Reilly replaces James Holt, who had held the role since 2014. He left the Lib Dems last week to work for US lobbying firm Mercury Public Affairs, as its lead in the UK.
Insiders say Reilly’s key tasks in the months ahead will be to capitalise on the new surge in membership since the EU referendum result, as well as taking advantage of divisions within the Labour Party and regaining traction with the media with just eight MPs.
In an internal email to Lib Dem staff today, seen by PRWeek, the party's leader Tim Farron said: "I've known Phil for years and I am certain he will be absolutely brilliant. He has always worked well with people from all across the party and has brought energy and clarity to our message and media strategy – and a sense of humour too. I'm chuffed to have him back in HQ as part of my team as we lead the Lib Dem fightback in the months ahead."