At the first elections for the New South Wales Legislative Assembly after the granting of responsible government in 1856, Brenan was an unsuccessful candidate for the two member seat of Cumberland (South Riding). He was defeated by William Manning and Elias Weekes. However, Weekes had also been a candidate for the seat of Northumberland Boroughs and although initially defeated in that electorate, he successfully contested the result and was declared to be the member. Unable to represent two electorates, Weekes chose to represent Northumberland Boroughs and Brenan was successful at the subsequent Cumberland (South Riding) by-election.
