Lieutenant Commander Tarah Johnson is a native of Phoenix, Arizona, and was raised in Vernon, Texas. She graduated as salutatorian from Vernon High School in 1990 where she played varsity volleyball. Tarah attended college at Grand Canyon University in Phoenix, graduating in 1994 with a Bachelor of Science degree in biology. She earned a Doctor of Osteopathic Medicine degree in June 1998 from Kirksville College of Osteopathic Medicine in Kirksville, Missouri.

Tarah was commissioned as a lieutenant in May 1998 and was accepted into the Navy's Aerospace Medicine Residency. As part of her residency, she completed an internship in family practice at the University of Alabama at Birmingham in June 1999. Tarah then earned a Master's Degree in Public Health from Johns Hopkins School of Public Health in Baltimore, Maryland, in May 2000.

Tarah reported to the Naval Aerospace Medicine Institute in Pensacola, Florida, where she received her naval flight surgeon wings in July 2001. She was awarded her board certification in Aerospace Medicine in November 2001 and completed residency training in June 2002.

In July 2002, Tarah reported as the flight surgeon to Marine Strike Fighter Training Squadron 101 (VMFAT 101) "Sharpshooters" at Marine Corps Air Station (MCAS) Miramar in San Diego, California. While there, she provided medical care to more than 530 personnel at the Marine Corps' largest squadron.