Devlin was educated at Harold Boys National School, Dalkey, Presentation Brothers College, Glasthule, and the Institute of Public Administration (Ireland).
Devlin became active in politics aged 16. He campaigned for Mary McAleese during her 1997 Presidential campaign, before joining Ógra Fianna Fáil in 1998. In 1999, he was elected to the Board of Dún Laoghaire VEC, he remained a member of the board until 2014.
Devlin, aged 23, was elected to Dún Rathdown Rathdown County Council in 2004, taking the second seat in the Dún Laoghaire local electoral area with 1,776 first preference votes and becoming the youngest ever councillor on the council.
Devlin was re-elected to a second term on Dún Laoghaire Rathdown County County, taking the fifth seat in the Dún Laoghaire local electoral area with 1,331 first preference votes. He was the only Fianna Fáil candidate to be elected in the Dún Laoghaire Dáil Constituency. The election was marked by some controversy. Throughout the campaign it was reported that then Minister Mary Hanafin was attempting to "shaft" Devlin. Her support for his Fianna Fáil rival, Peter O'Brien, was reported to include the improper used Oireachtas facilities in breach of Public Office Act 2001's Code of Conduct for Office Holders.
Devlin was re-elected to a third term on Dún Laoghaire Rathdown County County, coming second out of twenty one candidates.
In March 2015, at an event attended by over 100 supporters, Devlin announced that he would be seeing a Fianna Fáil nomination to contest the next general election in Dún Laoghaire. He was endorsed by former Minister Mary O'Rourke.
Former Minister Mary Hanafin and Kate Feeney also indicated their intention to seek the nomination. The contest attracted widespread media attention being dubbed the "Battle of Blackrock II".
In early September 2015, there was intense speculation the Fianna Fáil National Constituencies Committee would attempt to exclude Devlin from the Dún Laoghaire candidate selection convention on the basis of his gender. Following the threat of legal action from Devlin, the committee backed down.
Devlin was selected as a general election candidate on 28 September 2015, beating Mary Hanafin and Kate Feeney. Prior to the election, Hanafin was added to the general election ticket. The two candidates split the Fianna Fáil vote and the party failed to win a seat in the election.
Devlin was elected as Cathaoirleach of Dún Laoghaire Rathdown County Council on 13 June 2016, he listed housing, promoting civic engagement and supporting local enterprise as his key priorities during his term of office. In November 2016, Devlin took to a wheelchair for a day as part of a challenge to raise awareness of the issues facing people with a mobility impairment. He called on other public representatives to consider taking the challenge.