At age 18, Cupito changed her name to Morgan Brittany and then appeared with Gene Kelly in his Las Vegas show, Gene Kelly's Wonderful World of Girls, as a dancer. She then moved to New York City where she modeled for The Ford Modeling Agency, and appeared in several TV commercials and print ads (including a three-year stint as "The Ultra Brite Toothpaste Girl"), and was spokesmodel for brands such as L'Oreal, Ford, Levi's and Camay. In 1974, she was hired by the Japanese cosmetics company Kanebo to be the "face" of their product "Ireine", and so moved to Tokyo for two and 1/2 years. In 1976, she returned to the U.S. and restarted her acting career.
She appeared again as Leigh in the climax of the made-for-TV movie, The Scarlett O'Hara War in 1980, which marked the second of her three portrayals of Leigh. This caught the attention of the producers of Dallas, who were searching for an actress to play the sister of Pamela Ewing, Katherine Wentworth. Brittany debuted on Dallas in the 1981-1982 season. Her starring role as the sister of Pamela Ewing and Cliff Barnes continued for several seasons. In 1985, she returned for a cameo in that season's finale where she killed Bobby Ewing by running him over with a car. In a 1987 episode, after it was revealed that the previous season had been a dream, Brittany made a final brief return to the show, in which Katherine visited Pam in the hospital after a car accident. After her disappearance with Pam, Brittany went on to do other television shows and films.
Brittany married stuntman Jack Gill in 1981. They are the parents of actress Katie Gill and musician/stuntman Cody Gill.
During the 1980s, Brittany was a national ambassador for the March of Dimes. She traveled the United States raising funds and bringing awareness to the prevention of birth defects. She participated in the Congressional "Walk" in Washington, D.C., with such members of the House of Representatives as its then Speaker, Representative Thomas Phillip "Tip" O'Neill Jr., a Massachusetts Democrat. Her time with the March of Dimes lasted two decades, and during her involvement she was invited to the White House in 1986 with Ronald Reagan and again with Bill Clinton. Brittany splits her time between Southern California and Atlanta, Georgia.