McCaig was first elected to Aberdeen City Council in 2007, topping the polling in the Kincorth/Loirston ward with 1,883 first preferences and taking the first seat exceeding the quota.[3] Following the election, the Scottish Liberal Democrats formed a coalition with the SNP to run the Council. On 23 May 2011, McCaig took over as leader of the SNP group on the council.[4] After two by-election victories during 2011, the SNP became the largest party on the council, and in June 2011, their then Council leader stepped down to become Deputy Council leader.[5] As a result, McCaig, who had been acting as Deputy Council leader, became leader of Aberdeen City Council on 28 June 2011. At the time, aged 26, he was one of the youngest council leaders in the United Kingdom.[6]
He was re-elected as a councillor in 2012. After the elections, he remained the group leader of an enlarged SNP delegation, but was no longer the council leader, after the Scottish Labour group formed an administration with the support of the Conservatives and independents.[7]