He was raised on the family farm in the rural community of North River which was amalgamated into the town of Cornwall in 1996.
MacKinley has served as Opposition Critic for Transportation and Public Works, Agriculture, Forestry, Fisheries and Aquaculture; he was the Chairman of the important Public Accounts Committee of the legislature, which must always be chaired by a member of the Opposition.
On 12 June 2007, MacKinley assumed office as Minister of Transportation and Public Works, serving under PEI Premier Robert Ghiz.
His position was renamed to Minister of Transportation and Infrastructure Renewal during a government reorganization on 13 January 2010.
MacKinley announced his retirement from politics on 10 February 2015, choosing not to seek re-election in the 2015 provincial campaign.