Costello attended St Catharine's College, Cambridge, where he was awarded a degree in Experimental Psychology and qualified as a doctor in Medical Sciences after clinical training at the Middlesex Hospital in London.

After living in Baglung district in western Nepal from 1984 to 1986, two days' walk from a road, he became interested in challenges to mother and child health in poor, remote populations.

His areas of scientific expertise include the evaluation of cost-effective interventions to reduce maternal and newborn deaths, women's groups, strategies to tackle malnutrition, international aid and the health effects of climate change.

[3] With a Nepali organisation (MIRA), that he helped to establish, a large community trial of participatory learning and action using women's groups in the remote mountains of Makwanpur District, Nepal was published in The Lancet in 2004.

Seven cluster randomised controlled trials of women's groups in Nepal, India, Bangladesh and Malawi, led to a meta-analysis published in the Lancet in May 2013.

[6] Costello has led teams involved in more than a dozen cluster randomized controlled trials to show the power of community mobilization to affect health outcomes such as maternal and newborn deaths, child nutrition and diabetes.

[7] The book explains why a new science of cooperation is needed and suggests twenty two social experiments which use sympathy groups for resolving 21st century problems.

The Network aims to improve care in Ethiopia, Nigeria, India, Bangladesh, Malawi, Côte d'Ivoire, Uganda, Tanzania and Ghana.

[13] In 2020, based on this commission, he led the development of Children in All Policies 2030 which has set up links in Argentina, Senegal, Ghana, South Africa, India, Nepal and the Pacific Islands.

The group, chaired by Sir David Anthony King, a former Government Chief Scientific Advisor, was formed in early May 2020 to "provide a clear structure on which an effective policy should be based given the inevitability that the virus will continue to cross borders".

In April 2011, Costello received the James Spence Medal, the highest honour of the Royal College of Paediatrics and Child Health, where he is a fellow.