He continued to work on nutrient absorption and metabolism as a post doc at York University, followed by a year at the Swiss Federal Institute of Technology in Zurich studying the enzyme lactase. He then returned to York and changed research direction to examine secretion by the female reproductive tract and following a sabbatical at Harvard USA in 1980, began work on metabolism in early mammalian embryos. He devised and pioneered the use of non-invasive biochemical assays to determine the metabolism of single embryos and is co-inventor of four patents on the use of metabolic profiling to determine cellular health.   He was given a Personal Chair at York in 1997. He has been awarded Honorary Fellowships by the Association of Clinical Embryologists (ACE: 2002), British Fertility Society (BFS: 2010) Society for Reproduction and Fertility (SRF 2011) and was made a Fellow of the Royal College of Obstetricians and Gynaecologists (FRCOG ad eundem in 2003. He was awarded the Marshall Medal of the SRF in 2010. In 2016, he gave the BFS Steptoe Lecture and received Honorary Membership of the European Society of Human Reproduction and Embryology (www.youtube.com/watch?v=yAipBwYYVyQ). From 1998-2002 he was a member of the Human Fertilisation and Embryology Authority (HFEA) and is currently on their Horizon Scanning Group. 

Research interests include the biology of early mammalian embryos and their environment in the fallopian tube and uterus. Professor Leese is also associated with Dr Roger Sturmey's research group within HYMS, which studies how preimplantation embryo metabolism relates to the capacity to produce a blastocyst and give rise to a pregnancy; specifically, it aims to identify the optimal metabolic zone within which embryos with maximum developmental potential will be located. This information is important in ensuring the safety and efficacy of techniques for human assisted conception and the health of the offspring in later life.