McCallister was born in Shreveport, Louisiana, in 1952. He remained in his home state to attend college at Louisiana Tech University in Ruston, graduating in 1974. At the time McCallister completed his degree, Humana was less than 15 years old. The company, originally called Extendicare, was formed in 1961 by two young lawyers and other investors who developed, owned, and operated a nursing home business. For the remainder of the 1960s the company acquired more nursing homes and expanded its business to include hospitals. By 1974 Extendicare had purchased about 10 hospitals and changed its name to Humana.
McCallister joined Humana in 1974 as a finance specialist, working in the company's headquarters in Louisville, Kentucky. After a year he returned to Louisiana to serve as executive director of finances for Community Hospital in Springhill, which was a branch of Humana. In 1978 McCallister moved again, accepting a position as executive director for Humana hospitals in Huntington and West Anaheim in California. He remained there from 1978 through 1985, during which time he earned an MBA degree from Pepperdine University in Malibu, California.
Humana grew rapidly throughout the 1970s. By the early 1980s it was the world's largest hospital company, owning more than 80 hospitals. McCallister saw the company's growth at first hand, which later helped him deal with change inside the corporation after he moved into upper-level management positions. In 1985 he moved to Humana Hospitals in Canoga Park, California, and served as executive director there until 1988.
McCallister became the president of Humana Hospital in Phoenix, Arizona, in 1988. During the following year, he was elected to serve as Humana's vice president for Humana Health Care Plans in Phoenix, where his job required him to integrate health care and hospital plans. McCallister was transferred to San Antonio, ...