Mhairi was elected as the Member of Parliament for Paisley & Renfrewshire South in May 2015, when she beat the Labour Shadow Foreign Secretary, Douglas Alexander.

Mhairi is the current Baby of the House as the youngest member in the House of Commons of the United Kingdom. When elected in May 2015, she was 20 years and 237 days old, making her the youngest MP elected to the House of Commons of the United Kingdom since at least the Reform Act of 1832, replacing James Dickson who was 21 years and 67 days old when  elected in 1880.

At the time of her election, she had not yet completed her undergraduate degree, with a final exam on Scottish politics still to be undertaken. She successfully graduated with a first class honours degree in Politics and Public Policy from the University of Glasgow. Mhairi made her maiden speech on 14 July 2015 and this included some criticism of the government’s approach to unemployment in her constituency and the growing need for food banks. She said, “Food banks are not part of the welfare state. They are a symbol that the welfare state is failing.” Black also criticised the government over cuts to housing benefit. Her speech was praised by SNP Parliamentary Group Leader, Angus Robertson, who described it as outstanding, principled and passionate. Within five days of her giving this speech, it had been viewed over 10 million times on various media.