Brennan was born in Dayton, Ohio, in 1954, one of four daughters of James Richard Brennan and Virginia Lavern Rickey. Chrisann was named after the flower chrysanthemum. Brennan notes in her memoir that she is "dyslexic, which has had the effect of making me differently wired, creative, and a voracious Problem solver— bright, but more than slightly clueless to convention."

Brennan and Jobs relationship began in 1972 while they were students in high school together. Brennan remained involved with Jobs while he was at Reed College. Brennan (who was now a senior at Homestead High School) did not have plans to attend college, and was supportive of Jobs when he told her he planned to drop out of Reed. He continued to attend by auditing classes, but Brennan stopped visiting him. Jobs later asked her to come and live with him in a house he rented near the Reed campus, but she refused. He had started seeing other women, and she was interested in someone she met in her art class. Brennan speculates that the house was Jobs' attempt to make their relationship monogamous again.

In mid-1973, Jobs moved back to the San Francisco Bay Area. Brennan states by this point that their "relationship was complicated. I couldn’t break the connection and I couldn’t commit. Steve couldn’t either." Jobs hitchhiked and worked around the West Coast and Brennan would occasionally join him. At the same time, Brennan notes, "little by little, Steve and I separated. But we were never able to fully let go. We never talked about breaking up or going our separate ways and we didn’t have that conversation where one person says it’s over." They continued to grow apart, but Jobs would still seek her out, and visit her. They remained involved with each other while continuing to see other people.

By early 1974, Jobs was living what Brennan describes as a "simple life" in a Los Gatos cabin, working at Atari, and saving money for his impending trip to India. Brennan visited him twice at the cabin. Brennan's memories of this cabin consist of Jobs reading Be Here Now (and giving her a copy), listening to South Indian music, and using a Japanese meditation pillow. Brennan felt that he was more distant and negative towards her. Brennan states in her memoir that she met with Jobs right before he left for India and that he tried to give her $100 that he had earned at Atari. She initially refused to accept it but eventually accepted the money.

In early 1975, Brennan became involved with a Zen Buddhist community in Los Altos, where she accidentally bumped into Jobs (whom she had not seen since the All One Farm). It was through this community that they would both meet and work with the Zen master Kobun. Jobs and Brennan began to spend more time together, although she noted that his behavior with her was more aloof than in the past. Brennan notes their lives were on different paths as she was deeply involved in her art program at Foothill College where she studied under Gordon Holler while Jobs was working with "Kobun and Woz." She also fell in love with Greg Calhoun (Jobs' former Reed classmate) who had come to visit from the All One Farm. Brennan moved to the All One Farm and lived for a while with Calhoun in a renovated chicken coop. They eventually moved back to the Bay Area to earn money for a trek to India. Jobs helped them find a home to rent, though he was opposed to their traveling together to India. After Brennan and Calhoun had earned enough money to go, Jobs drove them to the airport and gave them advice about how to survive in India. Brennan traveled for a year through India with Calhoun, though their relationship ended by the time she returned to the United States.

Brennan's relationship with Jobs was deteriorating as his position with Apple grew and she began to consider ending the relationship. In October 1977, Brennan was approached by Apple employee #5, Rod Holt, who asked her to take "a paid apprenticeship designing blueprints for the Apples." Both Holt and Jobs felt that it would be a good position for her, given her artistic abilities. Brennan's decision, however, was overshadowed by the fact that she realized she was pregnant and that Jobs was the father. It took her a few days to tell Jobs, whose face, according to Brennan "turned ugly" at the news. According to Brennan, at the beginning of her third trimester, Jobs said to her: "I never wanted to ask that you get an abortion. I just didn’t want to do that." He also refused to discuss the pregnancy with her. Brennan, herself, felt confused about what to do. She was estranged from her mother and afraid to discuss the matter with her father. She also did not feel comfortable with the idea of having an abortion. She chose instead to discuss the matter with Kobun, who encouraged her to have and keep the baby as he would lend his support. Meanwhile, Holt was waiting for her decision on the internship. Brennan states that Jobs continued to encourage her to take the internship, stating that she could "be pregnant and work at Apple, you can take the job. I don’t get what the Problem is." Brennan however notes that she "felt so ashamed: the thought of my growing belly in the professional environment at Apple, with the child being his, while he was unpredictable, in turn being punishing and sentimentally ridiculous. I could not have endured it." Brennan thus turned down the internship and decided to leave Apple. She states that Jobs told her "If you give up this baby for adoption, you will be sorry" and "I am never going to help you."

At the age of 23, Brennan gave birth to her daughter, Lisa Brennan, on May 17, 1978. Jobs initially did not come for the birth. He eventually did so after he was contacted by Robert Friedland, their mutual friend and owner of the All One Farm. While distant, Jobs worked with her on a name for the baby. Brennan suggested the name "Lisa" and notes that Jobs was very attached to the name "Lisa" while he "was also publicly denying paternity." She would discover later that Jobs was preparing to unveil a new kind of computer that he wanted to give a female name. She states that she never gave him permission to use the baby's name for a computer and he hid the plans from her. Jobs also worked with his team to come up with the phrase, "Local Integrated Software Architecture" as an alternative explanation for the Apple Lisa (decades later, however, Jobs admitted to his biographer Walter Isaacson that "obviously, it was named for my daughter").

During the late 1980s, Brennan decided to finish her formal education and began to study at the California College of Arts and Crafts (where she was able to transfer her units from Foothill College). She asked Jobs to pay her tuition. He agreed to this request and according to Brennan was quite happy to do so, as part of his developing relationship with Lisa. In 1989, she transferred to the San Francisco Art Institute.

When Lisa was a baby and Jobs continued to deny paternity, a DNA paternity test was given that established him as Lisa's father. He was required to give Brennan $385 a month and return the money she had received from welfare. Jobs gave her $500 a month at the time when Apple went public, and Jobs became a millionaire. Brennan worked as a waitress in Palo Alto. Later, Brennan agreed to give an interview with Michael Moritz for Time Magazine. It would be for its 1982 Time Person of the Year special (released on January 3, 1983). She decided to be honest about her relationship with Jobs. The Time Magazine issue had a lifelong impact on Brennan. Rather than give Jobs the "Person of the Year" award, the issue was called "Machine of the Year: The Computer Moves In." In the issue, Jobs questioned the reliability of the paternity test (which stated that the "probability of paternity for Jobs, Steven... is 94.1%"). Jobs responded by arguing that "28% of the male population of the United States could be the father." Time also noted that "the baby girl and the machine on which Apple has placed so much hope for the Future share the same name: Lisa." After this issue, Brennan "didn’t pay much attention to Steve’s career again."

According to Fortune, Brennan wrote a letter to Jobs in 2005, and another in 2009, in which she said she would abandon writing her memoirs if Jobs would supply her with financial compensation of $28 million USD for the suffering she went through as a single mother.

Brennan is currently a Painter who lives in Monterey, California. She describes her art as "light encoded paintings" and works mostly on commission for either private or corporate parties. She has also created murals for the Ronald McDonald House, Los Angeles County Hospital, Massachusetts General Hospital in Boston, and Packard Children’s Hospital. Brennan states that painting is "a language for me. Letters are form, and paintings are documents for information. When I mix those two, I’m happy. It gets me out of the normal way, which is what I want to do."