J.J. was born in Waymart, Pennsylvania. His father, James Brown, was an Irish immigrant. His mother, Cecilia Palmer, was a schoolteacher. J.J's family moved to Pittston, Pennsylvania shortly after his birth. J.J's mother home-schooled him and later sent him to St. John's Academy. He left home at the age of 23, lured by the riches in the West. His first stop was a farm in Nebraska. From Nebraska he moved to the placer mines in the Dakotas. He spent two years learning the mining trade before moving to Colorado where he tried his luck in Georgetown, Aspen, and Ashcroft. He spent another two years in the Aspen and Ashcroft area before moving to Alma, Fairplay, Red Cliff, and, finally, Leadville. He studied geology, ore deposits, and mining techniques to become a better miner.
J.J. married Margaret Tobin on September 1, 1886, in Leadville's Annunciation Church. Margaret was later known as The Unsinkable Molly Brown when a Broadway fictionalized movie about her life titled, The Unsinkable Molly Brown was released in 1964. That was how Margaret's moniker Molly was acquired. Margaret Brown was one of the survivors of the RMS Titanic sinking in 1912. At the time of their marriage, J.J. was 31 and Margaret was 19. They first settled in Leadville, Colorado in a small, two-room log cabin, then later moved to a two bedroom house at 320 East Ninth Street. The Browns had two children. Their first child, Lawrence Palmer Brown, nicknamed Larry, was born on August 30, 1887 in his maternal grandparents' four room cottage near the Mississippi River in Hannibal, Missouri. Larry married Eileen Elizabeth Horton (1890–1985) on January 1, 1911 in Kansas City, Missouri. They had two children, Lawrence Palmer "Pat" Brown Jr. (1911–1976) and Elizabeth "Betty" Brown (1913–1974). The marriage failed and Larry married Hollywood actress Mildred Gregory (1895–1956) on November 17, 1926 in Beverly Hills, California. This marriage produced no other children. Larry died on April 2, 1949. J.J. and Margaret's second child, Catherine Ellen Brown, nicknamed Helen, was born on July 22, 1889 in her parents' three bedroom house at 322 West Seventh Street in Leadville, Colorado. She married George Joseph Peter Adelheid Benziger (1877–1970) on April 7, 1913 in Chicago, Illinois. Her children were James George Benziger (1914–1995) and George Peter Joseph Adelrich Benziger (1917–1985). Her grandchildren include Bradford Benziger (born December 13, 1944), Katherine Benziger (born July 12, 1947), Vincent Benziger (born August 3, 1950), Pamela Benziger Feeley (born 1946), Alice Lynne Davy (born 1948), Helen Anne Benziger (born 1950) and Heidi Rautio (born 1952). Helen died on October 17, 1969.
In 1909, after 23 years of marriage, J.J. and Molly signed a separation agreement and went their separate ways. The agreement gave Margaret a cash settlement and possession of the Victorian mansion on Pennsylvania Street in Denver's Wealthy Capital Hill neighborhood, and also the summer mansion Avoca Lodge in Southwest Denver, near Bear Creak. She also received a $700 monthly allowance to continue her travels and philanthropic activities. Although they never reconciled, they remained connected and cared for each other throughout their lives. At the time of J.J.'s death in 1922, Margaret told newspapers, "I've never met a finer, bigger, more worthwhile man than J.J. Brown."
Although J.J. was not born into wealth, he was ambitious, smart, and charismatic. He quickly rose through the mining ranks to shift-boss and timberman. He was foreman of the Louisville Mine by the time he and Margaret met. By 1887 he was superintendent of the Louisville Mine, and in 1888 he was superintendent of the Henriette & Maid Consolidated Mining Company, at the time one of the most productive mines in the area.
The Brown family came into great wealth when J.J's engineering efforts proved instrumental in the production of a substantial gold and copper seam at the Little Jonny mine of his employers, Ibex Mining Company.
J.J., who was the superintendent of all the Ibex properties, devised a method of using baled hay and timbers to stop cave-ins. His invention paid off. When the Little Jonny mine opened, vast quantities of high-grade copper and gold were found. The grade of gold was so pure and the vein so wide that it was heralded as the world's richest gold strike. By October 29, 1893, the Little Jonny was shipping 135 tons of gold ore per day. J.J. was awarded 12,500 shares or 12.5% of stock and a seat on the board. The Ibex Company and its owners, including the Browns, became extraordinarily wealthy. In 1894, the Browns moved to Denver, Colorado, buying a $30,000 Victorian mansion in Denver's wealthy Capitol Hill neighborhood. In 1897, they built a summer mansion Avoca Lodge in Southwest Denver, near Bear Creak.
On September 5, 1922, J.J. died after suffering a series of heart attacks at a hospital in Nassau, New York, with his daughter Catherine by his side. He died without a will, and it took 5 years of fighting between Margaret and her two children to settle the estate. Due to their lavish spending, J.J. left an estate valued at only $238,000 (worth $3,405,340 today). Margaret was to receive $20,000 in cash and securities, and the interest on a $100,000 trust fund set up in her name. $118,000 was divided between her two children. Her son Lawrence received a $59,000 (worth $689,368 today) trust fund, and her daughter Catherine received a $59,000 trust fund. From that time until her death in 1932, Margaret had no contact with her two children (but Margaret's page says they were reconciled). Both J.J. and Margaret are buried in the Cemetery of the Holy Rood in Westbury, New York.