De Rosenroll's prominence was such that, when Wetaskiwin received a seat in the Legislative Assembly of the Northwest Territories beginning with the 1898 election, he was acclaimed to it. He was re-elected by acclamation during the 1902 election. When Alberta became a province in 1905, de Rosenroll ran in its first election in Wetaskiwin as the Liberal candidate. He defeated Conservative R.W. Angus handily. When the government introduced a bill increasing the size of the legislature from sixteen seats to 41, de Rosenroll argued that not enough of the new seats were going to rural regions. He did not seek re-election in the 1909 election.