Markey was born and raised in Tulsa, Oklahoma. He attended Oral Roberts University and graduated with a degree in biomedical chemistry. He performed bioinformatic research as a post-baccalaureate fellow in the lab of Kevin Gardner at the National Cancer Institute, National Institutes of Health before moving to Dallas to attend medical school. Markey joined the UT Southwestern Medical Scientist Training Program in 2003 and received his MD and PhD degrees in 2010. He performed his PhD research in the laboratory of Jay Horton studying the role of PCSK9 in the molecular mechanisms that control cholesterol levels in humans. After graduating, he continued his clinical training completing a residency in internal medicine and fellowship in endocrinology, diabetes, and metabolism as part of the Physician Scientist Training Program at UT Southwestern. In fellowship, he worked in the laboratory of Helen Hobbs and Jonathan Cohen trying to better understand the metabolism of fatty acids in fasting and refeeding. During this time, Markey became passionate about rare genetic metabolic disease and returned to training to complete a residency in clinical genetics at UT Southwestern before joining the faculty as an assistant professor of genetics and metabolism in the McDermott Center for Human Growth and Development, Department of Internal Medicine, and Department of Pediatrics. He is board certified in Internal Medicine, Endocrinology, and Clinical Genetics. He splits his efforts between seeing patients in the clinic, performing translational research into rare and undiagnosed genetic diseases, performing clinical trials for new treatments of rare metabolic disease and leading educational activities for medical students, graduate students, and medical residents and fellows.