Beaven was elected to the provincial legislature in 1871, the year of union with Canada, and served in the cabinets of De Cosmos and his successor George Anthony Walkem as chief commissioner of land and works. He was accused by the opposition of corruption and was criticised for not opening up land for settlement quickly enough as well as running up the deficit. Beaven was in opposition during the government of Andrew Charles Elliott but rejoined the government when Walkem became premier for a second time in 1878 and became minister of finance and agriculture. Due to a shortage of revenues he instituted a poll tax in 1881. In 1882 Beaven became premier of the province but many of his supporters were defeated in the subsequent July election and Beaven ruled with a minority.
In January 1883 Beaven attempted to introduce a legislative program but his government was brought down by a Motion of No Confidence and he resigned on January 29, 1883.
Following the 1898 election, no party had a majority and the lieutenant-governorThomas Robert McInnes passed over the former premier John Herbert Turner and asked Beaven to form a government despite the fact that he had been out of the legislature for four years. Leading members of the legislature refused to support Beaven and he was unable to form a cabinet.
Robert Beaven died in 1920 and was interred in the Ross Bay Cemetery in Victoria.