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Not long after being diagnosed with Stage 2 breast cancer in December 2015, Chris-Tia Donaldson felt a sudden rage at her fate. A Harvard-educated lawyer, she was just starting to succeed in business after a long struggle as the founder of a hair-lotion company. Now she suddenly found herself sidelined by a terrifying disease.

Tempted to throw her iPhone at the nearest wall, she remembered it would be a nuisance to replace or repair the device. So she did the next best thing—stomping into her bathroom and kicking the glass shower door in what she described as Bruce Lee style.

It shattered. She lay down and sobbed. “It was exactly what I needed,” she wrote in her 2019 memoir, “This Is Only a Test.”

Ms. Donaldson underwent chemotherapy and radiation without giving up her role as chief executive officer of Thank God It’s Natural, or tgin, a Chicago-based maker of hair and skin-care products. She nurtured what had been a tiny home-based business into a supplier to national store chains including Target and Rite Aid. Her company specializes in products for Black women who want a natural hair look.

During her recovery, she set up the tgin Foundation to help women who, unlike her, couldn’t afford cancer care at the best hospitals. Just the cost of getting to the hospital or clinic for frequent treatments—Uber fares, parking fees, child-care expenses—could be crushing for low-income women, she said. Dreary waiting rooms, she believed, made it hard for patients to feel hope.

Her cancer returned, and Ms. Donaldson died Nov. 13. She was 42.

Chris-Tia Emon Donaldson was born Jan. 20, 1979, and raised in Detroit. Her mother, Marie Farrell-Donaldson, the first Black female certified public accountant in Michigan, served as auditor general and ombudsman for Detroit’s city government. Her father, Clinton L. Donaldson, was a commander in the police department.

They sent her to a predominantly white private school in the suburbs, then to an all-girl high school with a more diverse student body. Whether it was algebra or pull-ups in gym class, she strove to be No. 1. In the arena of hair, however, she felt that her kinky curls left her at a major disadvantage. In her memoir, she recalled yearning for long, blond tresses that could be pulled back into a scrunchie.

Shortly after she graduated from high school, her mother died of non-Hodgkin lymphoma. Ms. Donaldson soldiered on with her plans and enrolled at Harvard, where she earned a bachelor’s degree in economics and a law degree.

By the time she finished law school, she had resolved to give up chemical relaxers and let her hair go natural. When she joined the Chicago office of the law firm Sidley Austin, however, she feared her natural look might offend white colleagues and hurt her career chances. So she wore wigs.

To be accepted by white colleagues, Ms. Donaldson wrote later, she felt the need to be “a younger version of the ultra beautiful, super smart and sophisticated Clair Huxtable,” a character in “The Cosby Show” TV series.

While baking teriyaki wings for a party, she leaned close to the oven and singed the bangs of her wig, turning them into a plastic blob. From that moment, she wrote later, she decided to “embrace my natural hair, once and for all.”

After getting a negative performance review at Sidley Austin, she hopped to another law firm, Jenner & Block. From there she moved to Oracle Corp.

, where her duties included negotiating license agreements.

Meanwhile, in her free time she was testing her entrepreneurial skills. Her research into how to care for natural hair led her to write a book, “Thank God I’m Natural: The Ultimate Guide to Caring for and Maintaining Natural Hair,” published in 2009.

She mixed her own hair and skin potions, sometimes using a hand-held Oster mixer more suited for making brownies, and sold them from the back of her Ford Escape at farmers markets and festivals. As her book caught on, she won a reputation as an expert on natural Black hair care. A Chris Rock documentary on Black women’s hair struggles, “Good Hair,” encouraged more women to go natural, a boost for Ms. Donaldson’s emerging tgin brand.

Mariano’s grocery stores in the Chicago area began stocking her products in 2014. The next year brought a much bigger expansion when Target stores began displaying tgin products, including Honey Miracle Hair Mask. A perfectionist, she often inspected bottles one by one before shipment to make sure there were no crooked labels or other flaws.

In recent years, she began to delegate more and set aside her workaholic tendencies long enough for a vacation in Bali, Indonesia, where she discovered lychee martinis.

In her 2019 book, she wrote that her priorities included “working towards starting and building my family.” The return of cancer dashed those dreams.

Her survivors include her father and a sister, Dr. Piper A. Farrell.

Her legal training, she said, gave her an attention to detail and risk that may have saved her company from going bust in its early years. As a chief executive, however, she eventually decided she had to trust her intuition more often and make quick decisions. Leaders, she said at a Harvard panel discussion in 2016, must learn to “call it like an umpire: You’re either out or you’re safe.”

Danice Woodley, a friend since her Harvard days, recalled Ms. Donaldson as “radiant and effervescent” and “the most unapologetically who-she-is person I’ve ever met.” Ms. Donaldson gave up the law to run her own firm partly because “she didn’t want to conform any more,” Ms. Woodley said.

Write to James R. Hagerty at bob.hagerty@wsj.com