In 1853, Monro was elected to the 1st New Zealand Parliament, representing the seat of Waimea. He was re-elected in the same seat for the 2nd Parliament in 1858. At the beginning of the 3rd Parliament, to which Monro had been elected as representative of Picton, he was selected as Speaker. He is generally regarded as having conducted this duty "with dignity", although his use of the Speaker's casting vote to unseat Premier William Fox in 1862 was controversial. At the 1866 general election, he successfully contested the Cheviot electorate and declared elected unopposed. Monro remained Speaker until 1870, when he announced that he would step down. William Fox, who was once again Premier, failed to move the traditional vote of thanks.