what position did he assume first?
He was elected to Brandon's first city council.
Arthur Sifton
His first bid for elected office took place in 1882, when he was elected to Brandon's first city council. 
who was arthur sifton? A Canadian politician and the second Premier of Alberta. when was he born? Ontario. when did he start his political career? 1878.
wiki:11659894
Arthur Sifton [SEP] Early political career
Sifton's first foray into politics was in 1878, when he campaigned for the introduction of prohibition under the auspices of the Canada Temperance Act in the Manitoba electoral districts of Lisgar and Marquette. His first bid for elected office took place in 1882, when he was elected to Brandon's first city council. He was re-elected in 1883, and did not seek re-election at the conclusion of this second term, though he did briefly consider running for mayor before concluding that he had insufficient support to be elected. He also served on the local school board. When his brother Clifford became Wilfrid Laurier's Minister of the Interior in November 1896, Sifton advised him on Liberal Party affairs in western Canada.