Alcock was elected to the Legislative Assembly of Manitoba for the Winnipeg division of Osborne in the 1988 provincial election, in which the Manitoba Liberal Party rose from one seat to twenty under the leadership of Sharon Carstairs. He later worked as campaign manager for high-profile Liberal incumbent Lloyd Axworthy in the 1988 federal election. Alcock served as official opposition house leader and finance critic and was re-elected in the 1990 provincial election despite a shift against his party. He endorsed Jean Chrétien's bid to lead the federal Liberal Party in 1990, and declared his own intention to enter federal politics in 1992.