Following her marriage to Nelson in 1963, Harmon joined the Nelson family television show The Adventures of Ozzie and Harriet as a regular cast member, first appearing in the episode, "Rick's Wedding Ring".
In 1965, she co-starred with Nelson in the romantic comedy, Love and Kisses, in which they demonstrate the troubles of a young couple of school age who get married—an "inspired casting", according to one critic.
Subsequently, she played Officer Jim Reed's wife Jean on Adam-12, guest-starred on other series and appeared in a few theatrical films, including The Resurrection of Broncho Billy, which won an Academy Award for best live action short film.
She retired from acting in 1982 following Liar's Moon.
In 1988, she met the director-producer Mark Tinker, who asked her what she wanted to do with her life. She told him, "I want to paint."
She made her living as an artist, receiving a career boost when Jacqueline Kennedy purchased one of her paintings. She became a favorite of several Hollywood collectors including Mia Farrow, Tyne Daly and Dwight Yoakam. Her work, which is "widely acclaimed," is in the primitive genre, and has been likened to that of Grandma Moses.
Her paintings are conceived without perspective and are brightly colored with many figures included. Judy Blundell said, "Any symbolism is straightforward and honest. As an artist she is not concerned with being clever or elusive; she is simply using her talent as a means of true visual documentation."
Subjects include When the Kennedys Were in the White House (1964) and The Day He Died (1990), a memorial to her father which is painted on a window frame and depicts a country church and clouds raining. In 1999, Nelson's paintings were published in a coffee-table-sized autobiography, Out of My Mind. The paintings document her life story and are supplemented with diary entries and poems.
Harmon is the daughter of football star Tom Harmon and actress Elyse Knox. Her brother Mark is an actor and her sister Kelly is an actress-model.
Harmon began dating her longtime friend Ricky Nelson on Christmas Day, 1961.
A year later, the couple announced their engagement,. They were married on April 20, 1963, in a Catholic ceremony at St. Martin of Tours Catholic Church in Los Angeles. Harmon was pregnant, and Nelson later described the union as a "shotgun wedding." Nelson, a non-practicing Protestant, received instruction in Catholicism before the wedding and signed a pledge to have any children of the union baptized in the Catholic faith.
By 1975, the Nelsons were on the verge of breaking up. When Nelson returned from a tour in 1977, he discovered Harmon had moved him out of their home and into a rented house. Less than a month later, she found him there with two Los Angeles Rams cheerleaders. Nelson later claimed that she set him up to use the incident against him in court.
In October 1977, Harmon filed for divorce and asked for alimony, custody of their four children and a portion of community property, but the couple temporarily reconciled.
In April 1980, the couple bought Errol Flynn's 1941 Mulholland Drive estate for $750,000. Harmon wanted Nelson to give up music, spend more time at home, and focus on acting, but Nelson continued touring relentlessly. The dispute over Nelson's career created unpleasantness at home.
In October 1980, Harmon again filed for divorce. Attempts to negotiate a preliminary settlement agreement were unsuccessful. In February 1981, Harmon was temporarily granted custody of the children and $3,600 in spousal support. Nelson was required to pay a number of family expenses such as property taxes, doctor bills, and school tuitions. Harmon claimed Nelson was hiding assets, but in fact Nelson was almost broke. Accusations of drug and alcohol use and poor parenting were exchanged and, after two years of acrimony, the couple was divorced in December 1982. The divorce was financially devastating for Nelson with attorneys and accountants taking over $1 million.
Harmon and Nelson had four children. Their first, daughter Tracy Kristine Nelson, was born six months after the wedding on October 25, 1963, at St. John's Hospital in Santa Monica, California. She weighed four pounds, one ounce, and was slightly premature. As a pre-schooler, she appeared in Yours, Mine, and Ours with Lucille Ball. In her teens, she attended the exclusive Westlake School for Girls. During her parents' marital difficulties, she stayed with her father in the Mulholland Drive house.
Twin sons Gunnar Eric Nelson and Matthew Gray Nelson were born on September 20, 1967. Shortly after their father's death, they formed the band Nelson, which is still together today.
Their fourth child, Sam Hilliard Nelson, was born August 29, 1974. At six years, he was placed in the care of his maternal grandparents.
Nelson left his estate to his four children.
In 1987, two years after Nelson's death, Harmon was undergoing drug rehabilitation when her brother Mark Harmon and his wife Pam Dawber petitioned for custody of Kristin's youngest son Sam, on the grounds that Kristin was incapable of good parenting. Sam's psychiatrist testified that the thirteen-year-old boy depicted his mother as a dragon and complained about her mood swings and how she prevented him from being with his siblings.
Mark Harmon dropped the custody petition after his sister made allegations of cocaine use by Dawber. Kristin retained custody, although Mark Harmon was granted visitation rights.
She married Mark Tinker in 1988; they divorced in 2000.