Gabelli, the son of Italian immigrants, was born in The Bronx and went to Fordham Preparatory School, where he graduated in 1961. He has said he read market reports for fun when he was very young and that he bought his first stock when he was 13 years old.

Gabelli won a scholarship and graduated from Fordham University summa cum laude in 1965. He received his Master of Business Administration degree from Columbia Business School. At Columbia, he was taught by Roger Murray, noted value investing professor and co-author of the Fifth Edition of Security Analysis, The Graham & Dodd Value Investing Bible. Gabelli and his firm later launched the Graham & Dodd, Murray, Greenwald Award for Distinguished Value Investors. This award is presented yearly at his Annual Client Symposium.

In 1976, Gabelli formed Gabelli & Co., an institutional brokerage house, with borrowed funds and money he had accumulated trading his own account. Soon after Gabelli formed Gabelli Investors ( later GAMCO Investors) to manage money for clients. After sending a memo to a Barron's Editor touting a company he fancied, Chris-Craft Industries, Inc., Gabelli landed on the cover of this financial weekly. By 1981, GAMCO had 81 accounts and was managing about $33 million. Despite the rocky economy and stagnant stock market or the late 1970s and early 1980s, Gabelli made money for his clients each year. While Investors were seeking to get in on the ground floor of a company's growth, Gabelli preferred to cash in on a company's death. During the next few years, he invested in or recommended companies that were taken over by other firms or privatized.

In 1977, Gabelli started GAMCO Investors, Inc., an investment consulting firm esteemed within the financial industry as a provider of investment advice, alternative Investments and mutual funds, as well as institutional and private wealth management. Gabelli has created mutual funds that have consistently been recognized as among the industry’s top performers. His “Private Market with a Catalyst” is widely recognized for its visionary and innovative research methodology that focuses on individual stock selection by identifying firms selling below intrinsic value with catalysts to realize the value. Thirty-five years after founding the company, as chairman and chief executive officer, Gabelli is still actively involved in GAMCO’s everyday operations. He travels often on Business, but is looking forward to being in the city for Columbus Day, and says now that he’s returned to New York from a recent Business trip, the totality of his impending duties as grand marshal are starting to take effect.

Between 1978 and 1985, Gabelli's portfolios outperformed the Standard & Poor's index of 500 stocks each year and more than doubled the results of this S&P index in five of those years. Between 1977 and 1988, Gamco Investors' assets appreciated at an annual compounded rate of 28 percent, a rate exceeded by very few money managers. This money management entity had never lost money over a year and had met Gabelli's objective of ten percent annual return after taxes and inflation in all but two years. GAMCO Investors' equity assets, flush with cash invested by company pension funds, reached $1.6 billion in 1986.

By the mid 1980s Gabelli was managing over $350 million in client assets and compounding returns at more than 35 percent per year. While Business Week was touting the "Death of Equities" on its cover, Gabelli was quoted as saying "I don't need a rising market to bail me out." Gabelli's firm was doing all its own research, usually with the idea of identifying firms that might be candidates for leveraged buyouts: those with characteristics such as a large amount of cash on hand, underlying assets such as real estate, or a large block of stock in the hands of a company founder with no children. He also looked for companies in industries where new competition was difficult and cash flow was high. Once Gabelli selected such a stock, he was willing to wait for years until it appreciated. He began buying Cowles Communications, Inc., for Example, in 1977 at $14 a share and eventually became its biggest stockholder. In 1984, the company (now Cowles Media Co.) was privatized at $46 a share, for a total payoff to Gabelli's clients of $33 million. Another media winner was Chris-Craft, whose BHC Communications achieved almost a sixfold pretax gain in income over six years in the 1980s and in the 1990s assembled its own network, United Television. The relentless Gabelli was visiting about 50 companies a year to gain information and meeting the managers of more than 100 other corporations annually, as well as getting together with other portfolio managers to discuss ideas and reading about 20 trade journals, two or three newspapers, and a number of industry and company reports. He also was writing research reports for his brokerage customers and portfolio-manager and other professional-investor clients. "I read annual reports instead of novels," Gabelli told Jerry Edgerton of Money in 1986. As a narrowly focused investor with large holdings in a few companies, he was able to bring influence to bear on company management.

Gabelli's first investment vehicle for the general public, The Gabelli Asset Fund, launched in March 1986 as a no-load fund requiring a minimum of $25,000 to invest. Later, and today, this fund is available for a minimum investment of $1,000 and accepts IRA Investments without a minimum. The Asset Fund was later followed on by The Gabelli Equity Trust, a closed-end fund, which, at the time, was the largest equity offering on the NYSE. By the end of 1988, Gabelli's firm had three mutual funds—two run by himself—with combined assets of $650 million.

Mr. Gabelli has been one of the program's most generous and ardent supporters since its inception in 1991.

Gabelli established the Gabelli Distinguished Presidential Scholarship and is a lead supporter of the Presidential Scholars Program. In addition, he has served as a University Trustee and Trustee Associate and has endowed the Mario J. Gabelli Professorship in Finance in the Carroll School of Management. In 1995, the residence hall at 80 Commonwealth Avenue was named in honor of Mr. Gabelli.

In 1997, when ten Gabelli equity funds averaged a return of 31.7 percent, the best of any U.S. mutual fund group, Gabelli was honored by Morningstar, Inc. as the domestic equity fund manager of the year.

By 1998, Gabelli Asset Management Inc. was managing $16.3 billion. In February 1999, the company went public selling 6 million shares, or about twenty percent of the Common stock at $17.50 per share.

On January 10, 2000, Gabelli was inducted into the Barron's All-Century Team, their list of the most influential mutual fund industry portfolio managers.

In 2001, whistleblower Rufus Taylor III filed a civil lawsuit against several companies owned by Gabelli, alleging fraudulent practices in FCC auctions between 1995 and 2000. In those auctions, the government set aside cell phone licenses to be sold to small businesses. Taylor's lawsuit said that Gabelli used more than twelve "sham" startup companies to meet the requirements of a small Business applicant in the auctions and acquire the licenses at a small Business discount.

Indeed, LICT's FCC attorney stated that Gabelli complied with FCC requirements. The suit had alleged that the Gabelli-backed false "entrepreneurs" included an NBA player, a related party, a former partner of an accounting firm, an aerobics instructor, and even the caretaker of a vacation home in part owned by Gabelli. On July 12, 2006, the suit was settled when the bidding entities reportedly agreed to pay $130 million to settle the allegations.

Gabelli himself graduate from Fordham and holds an M.B.A. from Columbia University Graduate School of Business and honorary doctorates from Roger Williams University in Rhode Island and Fordham University. He is a trustee of the Columbia University Graduate School of Business, Boston College and Roger Williams University. He also serves on the boards of The American-Italian Cancer Foundation, The Foundation for Italian Art & Culture, The Winston Churchill Foundation and the E.I. Wiegand Foundation. In 2010, he donated $25 million to Fordham University, the largest gift in the school’s history, for its undergraduate Business program, known now as the Fordham University Gabelli School of Business.

In 2011, a gift from Gabelli enabled the School of Business Administration to establish a new endowed professorship. The Gabelli Asset Management Endowed Professorship will provide critical support of the University’s faculty, research and services initiatives. In addition the Finance Professorship, GAMCO has supported the school’s academic initiatives, scholarships, mentoring and job placement initiatives.

On April 27, 2012, Mario J. Gabelli was at the 24th annual Boston College Wall Street Council Tribute Dinner in New York City’s Waldorf-Astoria. Mario was presented with the President’s Medal for Excellence in recognition of his achievements, exemplifying the Boston College motto “Ever to Excel.” The black-tie dinner drew more than 1,000 guests.

On June 27, 2013 at the Foreign Policy Association's 95th Anniversary and Annual Dinner, Mr. Gabelli was awarded the Foreign Policy Association Medal which recognizes individuals who demonstrate responsible internationalism and work to expand public knowledge of international affairs.

At its annual dinner on October 7, 2014, the New York Society of Security Analysts awarded Mario J. Gabelli the Irving Kahn Lifetime Achievement Award. This award is presented to a member of the investment profession who has made distinctive contributions to the industry and whose exemplary achievement, execellence of practice, and true leadership have elevated the integrity and competence of the profession. Gabelli's role as a leading proponent of the Graham & Dodd, Murray, Greenwald value investing methodoloy has made him an industry leader.

On October 17, 2015, Gabelli was honored by the National Italian American Foundation (NIAF) at their 40th Anniversary Gala in Washington, D.C. with their Leadership and Service Award recognizing him as an Italian American who has distinguished himself in Business and made a significant contribution to NIAF. Gabelli is a former member of the Board of Directors of NIAF.

“I did talk about it this morning with my wife,” Gabelli says. “I think the last time I walked down a parade was probably when I was in high school. The Columbus Day Parade happens on a Monday when Wall Street’s open, so historically, I’ve never tried to take the day off from work, but am more than delighted to do it this year”.