James A. McCloskey, Jr., grew up in San Antonio, Texas. He entered Trinity University in San Antonio, where he majored in chemistry; he earned a PhD in analytical chemistry from Massachusetts Institute of Technology. After fulfilling his ROTC commitment by working for the US Army Chemical Corps, McCloskey returned to Klaus Biemann's lab at MIT, where he began his lifelong interest in and study of nucleosides/nucleotides, necessitating different types of mass spectrometers. He turned down the Karolinska Institutet for a job at Baylor College of Medicine in Houston, Texas. He began a twenty-year collaboration with Susumu Nishimura in Tokyo, Japan, and made his first of many trips there. His lab discovered the nucleoside Q. He began his part of the search for the roots of the tree of life, which consists of bacteria, eukaryotes, and archaea. McCloskey spent six months of a sabbatical at the National Cancer Research Institute in Tokyo before going to the University of Utah as a visiting professor. He decided to accept a full professorship there. McCloskey became secretary, vice president, then president of the American Society for Mass Spectrometry (ASMS).