In 1984 Mansueto founded Morningstar out of his home with $80,000. Morningstar went public in May 2005.
In 1988 Mansueto married Japanese American Rika Yoshida, a Morningstar analyst. They have three children and live in a newly constructed home in Lincoln Park estimated to cost $22 million to build. Mansueto keeps strict work hours, 8:30 a.m. to 5 or 6 p.m., with an occasional Saturday at the office. He enjoys early morning jogs along Lake Michigan and reading the occasional annual report. Mansueto's younger brother John died in 2003 of West Nile virus.
Mansueto purchased the financial magazines Inc. and Fast Company in June 2005. He was a limited partner in the publication of the entertainment magazine Time Out Chicago from 2005 to 2013.
As of June 2008, Mansueto owned about 60.4% of Morningstar Common stock. In 2011, his majority ownership of Morningstar gained him inclusion on the Forbes World's Billionaires list, with a net worth at time of publication of $1.6 billion.
As of December 2010, Mansueto was the only Chicagoan on the list of American billionaires pledging to give away half of their wealth, as part of The Giving Pledge by Warren Buffett.
In May 2008 it was announced that Mansueto and his wife Rika pledged $25 million to the expansion of the main University of Chicago library. The new wing, called the Joe and Rika Mansueto Library, designed by Chicago-based Architect Helmut Jahn opened to the public on May 16, 2011.
In September 2016, Morningstar announced that it had appointed Kunal Kapoor as chief executive officer, effective January 2017, with Mansueto becoming executive chairman at the same time.