Brennan first stood for election to Dáil Éireann as a Fianna Fáil candidate in Wicklow in the 1953 by-election created as a result of the death of his father Thomas Brennan, who was a Fianna Fáil TD for Wicklow from 1944 to 1954. While unsuccessful, he was returned at the following 1954 general election, where he was elected to the 15th Dáil, and was returned at the next four general elections. He topped the poll on each occasion, and by virtue of one of the largest votes at the 1957 general election, he managed to secure two seats for Fianna Fáil in the constituency, where he was joined by his running mate James O'Toole.
He served as Parliamentary Secretary to the Minister for Local Government from 1965 until he resigned on 8 May 1970. His resignation followed that of his boss, local government minister Kevin Boland, in sympathy with Neil Blaney and Charles Haughey, who had both been sacked from the cabinet in the Arms Crisis.
After being expelled from the Fianna Fáil parliamentary party in 1971, Brennan remained an independent and while closely aligned with Kevin Boland did not in fact join Boland's new party, Aontacht Éireann. He contested the 1973 general election as an independent candidate, but he lost his seat to Fianna Fáil's Ciarán Murphy.
He subsequently rejoined Fianna Fáil, and at the 1981 general election he was elected again as TD for Wicklow. He lost his seat at the February 1982 election, and was a Taoiseach's nominee to the Seanad but was re-elected to the Dáil at the November 1982 general election, ousting Ciarán Murphy. Brennan retired from politics at the 1987 general election.