Aileen was born in East Kilbride and grew up in her home town. She studied at the University of Edinburgh before completing a PhD. at the University of Central Lancashire. She joined the SNP in 2004 before spending five years living in Brussels and working as Head of Policy for Alyn Smith MEP.

In 2011 McLeod was elected to the Scottish Parliament on the South Scotland regional list. In 2014 she was appointed Minister for Environment, Climate Change and Land Reform. During this time she represented the Scottish Government at the 2015 United Nations Climate Change Conference which resulted in the Paris agreement.

In 2019 she was elected as a Member of the European Parliament for Scotland.

In Parliament, she is a full member of both the Committee on the Environment, Public Health and Food Safety and Committee on Constitutional Affairs. She is a full member of the Delegation for Northern cooperation and for relations with Switzerland and Norway and to the EU-Iceland Joint Parliamentary Committee and the European Economic Area (EEA) Joint Parliamentary Committee.

She also sits on the the European Parliament Intergroup on Climate Change, Biodiversity and Sustainable Development and the MEPs Against Cancer (MAC) Interest Group.