[2] In 1905, Mitchell was elected to Calgary City Council as an Alderman representing the original Ward 3, and was subsequently re-elected for four more terms.

In June 1911, the construction of Calgary City Hall was completed and it was opened by Mitchell, along with leader of the opposition Robert Borden.

As mayor, Mitchell hosted the Governor General of Canada Prince Arthur, Duke of Connaught and Strathearn, and Princess Patricia.

[2] During his time as Mayor, Mitchell led the city in staking a mineral claim on oil and gas resources in an attempt to provide a new source of revenue.

[2] Mitchell again became Alderman in 1915, however he resigned on November 22, 1915 to enlist as a member of the 82nd Battalion of the Canadian Expeditionary Force for active overseas service during the First World War.