His mother was the daughter of Mark Sullivan, Sr., a former Editor of Collier's and later columnist with the New York Herald Tribune newspaper. She was a short story author (under a pen name), and a reporter for The Washington Post. His parents eloped on June 8, 1933, and were married in Rockville, Maryland.

Jameson Parker, Jr. was born in Baltimore, Maryland, on November 18, 1947. He was the son of Jameson and Sydney Buchanan (née Sullivan) Parker. His father had been general legal counsel for the Parker family steel mill, an investment analyst, and government attorney (working first with the Maryland Public Expenditure Council and later with the United States Naval Reserve). In 1947, he was in private practice, but about to embark on a career as a diplomat with the United States Department of State.

Parker has been married three times. On July 19, 1969, he married Anne Taylor Davis in Fairfax County, Virginia, with whom he has one daughter. The two divorced on August 7, 1975 in Alexandria, Virginia. In 1976 Parker married Bonnie Dottley in New York City; the couple had three children. They divorced in 1992. Later that same year, Parker married Darleen Carr.

At Beloit College, he acted in student theater productions, and, while living in Washington, D.C., he landed a job with a production of The Great White Hope at the Arena Stage and then acted in theatrical productions of Caligula and Indians. After completing his degree at Beloit College in 1971, he performed in dinner theater and summer stock in the Washington, D.C., area.

In 1972, he moved to New York City, where he secured several television commercials and appeared in off-Broadway plays. He was cast as Dale Robinson in the daytime drama Somerset and created the role of Brad Vernon on One Life to Live. During this period, Parker guest-starred on the ABC series Family and Hart to Hart.

In addition, he played the leads in several CBS television movies: Women at West Point (1979), Anatomy of a Seduction (1979), The Gathering II (1979), The Promise of Love (1980), Callie and Son (1981), and A Caribbean Mystery (1983).

He became well-known by co-starring in Simon & Simon from 1981 to 1989. Thanks to the hit show's popularity, in 1985, Beloit awarded him its Distinguished Service Citation. With his Simon & Simon co-star Gerald McRaney he appeared in the theatrical movie Jackals, which Parker co-produced. After completing this movie, he returned to Beloit College to star in a live summer stock theatrical production as Brick in Cat on a Hot Tin Roof. In 1987, starred alongside Donald Pleasence and Alice Cooper in John Carpenter's horror movie Prince of Darkness.

Parker guest-starred on the ABC Walker, Texas Ranger as a corrupt cop. He appeared in the television movies Who is Julia? (1986), Dead Before Dawn (1993), and Violation of Trust (1991). He appeared on the sitcom Major Dad with his Simon & Simon co-star Gerald McRaney.

In fall 1992, Parker was shot in the left arm pit and right arm by a neighbor near his home after a Verbal altercation where Parker was defending his wife's honor after she and the shooter argued earlier in the day. He made a full recovery, and the neighbor was convicted of attempted murder and sentenced to nine years in prison.

Parker's last known acting work in the Business was in 2003-2004, after a four year hiatus, when he appeared in four episodes of JAG. Although he did voice over work as the narrator for the documentary "Endangered Species: California Fish and Game Wardens" in 2009, he seems to have effectively retired from acting without an official announcement. He now makes his living as a Writer for a variety of hard-copy and on-line magazines and is the author of the critically acclaimed memoir, An Accidental Cowboy.