Margaret and J.J. were married in Leadville’s Annunciation Church on September 1, 1886. They had two children: Lawrence Palmer Brown (1887–1949), known as Larry, and Catherine Ellen Brown (1889–1969), known as Helen.

The Brown family acquired great wealth when in 1893 J.J.’s mining engineering efforts proved instrumental in the production of a substantial ore seam at the Little Jonny Mine of his employers, Ibex Mining Company, and he was awarded 12,500 shares of stock and a seat on the board. In Leadville, Margaret helped by working in soup kitchens to assist miners’ families.

In 1894, the Browns bought a $30,000 Victorian mansion in Denver, Colorado, and in 1897, they built a summer house, Avoca Lodge in Southwest Denver near Bear Creek, which gave the family more social opportunities. Margaret became a charter member of the Denver Woman’s Club, whose mission was the improvement of women’s lives by continuing education and philanthropy. Adjusting to the trappings of a society lady, Brown became well-immersed in the arts and fluent in French, German, Italian, and Russian. Brown co-founded a branch in Denver of the Alliance Française to promote her love of French culture. Brown gave parties that were attended by Denver socialites, but she was unable to gain entry into the most elite group, Sacred 36, who attended exclusive bridge parties and dinners held by Louise Sneed Hill. Brown called her “the snobbiest woman in Denver”.

After 23 years of marriage, Margaret and J.J. privately signed a separation agreement in 1909. Although they never reconciled, they continued to communicate and cared for each other throughout their lives. The agreement gave Margaret a cash settlement, and she maintained possession of the house on Pennsylvania Street in Denver and the summer house, Avoca Lodge. She also received a $700 monthly allowance (equivalent to $19,919 in 2019) to continue her travels and social work.

Brown assisted in fundraising for Denver’s Cathedral of the Immaculate Conception, which was completed in 1911. She also worked with Judge Ben Lindsey to help destitute children and establish one of the United States’ first juvenile courts, which helped form the basis of the modern U.S. juvenile courts system.

The Titanic sank early on April 15, 1912, at around 2:20 a.m., after striking an iceberg at around 11:40 p.m. Brown helped others board the lifeboats but was finally persuaded to leave the ship in Lifeboat No. 6. Brown was later called “The Unsinkable Molly Brown” by authors because she helped in the ship’s evacuation, taking an oar herself in her lifeboat and urging that the lifeboat go back and save more people. Her urgings were met with opposition from Quartermaster Robert Hichens, the crewman in charge of Lifeboat 6. Hichens was fearful that if they went back, the lifeboat would either be pulled down due to suction or the people in the water would swamp the boat in an effort to get in. After several attempts to urge Hichens to turn back, Brown threatened to throw the crewman overboard. Sources vary as to whether the boat went back and if they found anyone alive. Brown’s efforts sealed her place in history, regardless.

Brown ran for the U.S. Senate in 1914 but ended her campaign to return to France to work with the American Committee for Devastated France during World War I.

At the time of J.J. Brown’s death on September 5, 1922, Margaret told newspapers, “I’ve never met a finer, bigger, more worthwhile man than J.J. Brown.” J.J. died intestate, and five years of disputation between Margaret and her two children were required to finally settle the estate. Due to their lavish spending, J.J. left an estate valued at only $238,000, equivalent to $2,903,290 in 2019. Molly was to receive $20,000 in cash and securities (equivalent to $305,487 in 2019), and the interest on a $100,000 trust fund (equivalent to $1,527,435 in 2019) in her name. The sum of $118,000 was to be divided between her two children, who each received a $59,000 (equivalent to $901,187 in 2019) trust fund. A court case against Catherine and Lawrence was settled privately, and Margaret and her children were reconciled at the time of Margaret’s death in 1932.

During the last years of her life, she was an actress. Margaret Brown died in her sleep at 10:55 p.m. on October 26, 1932, at the Barbizon Hotel in New York City, New York. Subsequent autopsy revealed a brain tumor. Her body was buried along with J.J. in the Cemetery of the Holy Rood in Westbury, New York, following a small ceremony on October 31, 1932, attended only by close friends and family. There was no eulogy.

In 1985, she was inducted into the Colorado Women’s Hall of Fame.