Schoomaker has held a wide variety of assignments. From 1979 until 1982, he was a research hematologist at Walter Reed Army Institute of Research. He served as Assistant Chief and Program Director, Department of Medicine, Walter Reed Army Medical Center, from 1982 to 1988; Medical Consultant to Headquarters, 7th Medical Command, Heidelberg, Germany, 1988–1990; Deputy Commander for Clinical Services, Landstuhl Army Regional Medical Center, Landstuhl, Germany, 1990–1992; Chief and Program Director, Department of Medicine and Director of Primary Care, Madigan Army Medical Center, Tacoma, Washington, 1992–1995; Director of Medical Education for the Office of The Surgeon General/HQ USAMEDCOM conducting a split operation between Washington, DC, and Fort Sam Houston, Texas, 1995–1997; and Director of Clinical Operations at the HQ USAMEDCOM, February to July 1997. From July 1997 to July 1999, he commanded the USA MEDDAC (Evans Army Community Hospital) at Fort Carson, Colorado. He attended the U.S. Army War College in Carlisle Barracks, Pennsylvania, from 1999 to 2000 followed by assignments as the Command Surgeon for the U.S. Army Forces Command (FORSCOM) from July 2000 to March 2001, and Commander of the 30th Medical Brigade headquartered in Heidelberg, Germany, from April 2001 to June 2002.