Dr. Timmons received his B.S degree from Davidson College in Davidson, NC, followed by M.D. and Ph.D. from the University of North Carolina at Chapel Hill, where he completed his doctoral dissertation under Dr. Neil Kirkman in the Division of Human Genetics and Metabolism. He completed a residency in Anatomic and Clinical Pathology at Northwestern Memorial Hospital in Chicago and served as Chief Resident his final year. He then did a two-year fellowship in Anatomic and Clinical Pediatric Pathology at The Children's Hospital in Denver under Dr. J. Bruce Beckwith.  Dr. Timmons joined the faculty of UT Southwestern Medical Center in 1988 in the Division of Pediatric Pathology, where he participates in the Surgical Pathology and Autopsy services.  He has been the recipient of three small-group teaching awards for pathology from UT Southwestern medical students.  Since 2002, he has served as Director of Resident Education for the Department of Pathology.  He has held the academic rank of Professor on the Clinical Scholar Track since 2002.

Dr. Timmons is active at the national level in both pediatric pathology and graduate medical education. He is the Immediate Past-President of the Society for Pediatric Pathology (SPP), having served on the SPP’s Executive Committee since 2013 and as President for the 2019-2020 term. He has been the recipient of two Presidential Awards for Meritorious Service from the SPP:  one in 2018 for his service as Secretary and Treasurer and one in 2012 for his activities as Fellowship Committee Chair, which included implementation of a national fellowship in-service examination.  He has served on the Leadership Council of the Program Directors Section (PRODS) of the Association of Pathology Chairs (APC), which is the national organization of pathology residency directors, from 2010 to 2019, including holding the office of Chair of the Program Directors Section from 2014 to 2017. He is also active on the APC/PRODS Working Groups on Autopsy and on Entrustable Professional Activities.  He served on the Accreditation Council for Graduate Medical Education (ACGME) Review Committee for Pathology, which accredits pathology residency and fellowship training programs, from 2013 to 2019, including as Vice-Chair and as a member of the ACGME's Pathology Residency Milestones Revision Working Group. He serves on the American Board of Pathology's Test Development and Assessment Committee for Pediatric Pathology, which prepares questions for the certification examinations.  He is also a member of the Graduate Medical Education Committee of the College of American Pathologists and was the 2017 recipient of the Resident Advocate Award from the College of American Pathologists.