Devlin was re-elected to a second term on Dún Laoghaire Rathdown County County following the 2009 local elections, 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 local electoral area. 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. In 2012, Councillors Cormac Devlin and Barry Ward's use of a Local Government Education and Training Fund was reviewed on foot of a complaint from Shane Hogan, a public servant and Labour Party member. Hogan claimed their use of the fund was a breach of Section 168 of the Local Government Act, 2001. The report found neither councillor had breached legislation, noting it was the responsibility of council administrators to ensure payments for training and education went through proper procedure. The report recommended Devlin repay €910 of the fees, which was since repaid. Mr. Hogan branded the report a “whitewash” and subsequently appealed the decision to SIPO. SIPO accepted the recommendations of the report.
Devlin was re-elected to a third term on Dún Laoghaire Rathdown County Council following the 2014 local elections, 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 the Dún Laoghaire constituency. 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. Councillor Devlin is considered to be "pro life", during the 2016 general election he told a Vincent Browne People's Debate he held pro-life views. Devlin finished in sixth place with 4,665 first preference votes, behind Labour's Carrie Smyth, and his party colleague Mary Hanafin.
Chairperson of Dún Laoghaire Rathdown County Council
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.
2020 general election
At the February 2020 general election, Devlin was elected as a Fianna Fáil TD for the Dún Laoghaire constituency.
