Joyce is Ambassador and former Chief Executive of Compassion in World Farming, the leading charity advancing the welfare of farm animals worldwide. She has led Compassion's work with intergovernmental organisations, such as the Food and Agriculture Organisation of the United Nations (FAO) and the World Bank.

Joyce played a key role in achieving the UK ban on sow stalls in the nineties and in getting recognition of animal sentience enshrined in the European Union Treaties.

Joyce is an expert on the issues around industrial animal agriculture, including farm animal welfare, and speaks and publishes widely on these issues. She has given evidence to the UK and New Zealand governments on genetic engineering of animal. In 2007, she was invited to make a presentation to the European Commission's European Group of Ethical Advisers on Science and New Technologies on the welfare aspects of cloning farm animals for food. Recently she has been invited to participate in two invitee-only workshop on sustainable farming, food and GHG emissions with the UK Government's advisory Committee on Climate Change.

Joyce addressed (and organised, with the RSPCA) the first conference on pig welfare in Beijing in 2005, and has co-organised several further conferences in China in 2008 and 2009.

Joyce initiated Compassion's 2005 Conference "From Darwin to Dawkins: the Science and Implications of Animal Sentience", which attracted over 600 delegates from 50 countries and resulted in two publications: "Animals, Ethics and Trade" (Earthscan), for which Joyce wrote the Introduction, and a special edition of the journal "Applied Animal Behaviour Science".

In 2010 publishers Earthscan commissioned Joyce to produce/edit the book "The Meat Crisis" with Professor John Webster as co-editor.

She represents Compassion on the Management Committee of the new alliance: Eating Better for a fair, green, healthy future which aims to reduce meat consumption in high-consuming populations and promote humane, healthy and sustainable diets.

Joyce has been vegan since 1975 and wrote the second vegan cookbook published in the UK: "Healthy Eating for the New Age: A Vegan Cookbook".