The daughter of a career military man, Principal encountered controversy early in life, and it did not abate after she entered the world of entertainment, playing Paul Newman's mistress in "The Life and Times of Judge Roy Bean" (1972). Caught up in the whirlwind of socializing with various media types did not enhance the young actress's career and for several years she stagnated in small roles in TV and feature films, including "Earthquake" (1974) and "I Will...I Will...for Now" (1975).

Partly in order to work, partly to understand the Hollywood scene better, Principal shifted gears and worked for several years as an agent, her best-known client being comedian Dick Martin. Then in 1978, she landed on the cast of the phenomenally popular, barbecue-flavored primetime soap, "Dallas". Cast as the usually virtuous, always long-suffering wife of equally heroic Texas oilman Bobby Ewing (Patrick Duffy), Principal enjoyed considerable media exposure, though the limitations of the role, combined with her breathy line delivery and modest histrionic capacity, left viewers enjoying the glamour of a personality rather than the craft of an actress. Principal was generally at her best in the show in romantic encounters with Bobby or arguments with his slimy sibling J.R. (Larry Hagman). Appearances on "Celebrity Challenge of the Sexes", "Pleasure Palace" (both 1980) "60 Years of Seduction" and "Women Who Rate a 10" (both 1981) served largely to enhance Principal's image of lightweight, demure serenity.

Leaving the series just as it stepped onto its inevitable downhill slope, the determined, feisty and now-experienced Principal campaigned for the title role in the TV-movie, "Mistress" (1987), in order to escape the confines of her "Dallas" image. A restless, energetic woman, Principal formed her own production company and began executive producing a series of TV-movies, including "Naked Lie" (1989) and "Sparks: The Price of Passion" (1990). While these and other efforts did not garner her any new critical acclaim as an actress, they (and the trilogy of self-help books she authored) suggest not only an increased confidence and savvy, but also that Principal's true forte may lie as a commercially-minded TV producer.