Now the corporate golden-boy’s career is in tatters. A Los Angeles Times story this past Sunday charges that Nanula, posing as a porn star called “Mr. Rich,” videotaped himself having sex with adult film actresses. The story followed up on explicit photos posted on a website called Thedirty.com.

According to the LA Times story, the fateful entanglements, along with a sexual harassment charge, led Nanula, 53, to resign in July from private equity firm Colony Capital and the Miramax film company, where he was chairman.

I knew Nanula back when he was at his height of success. And ironically, he was known as an extraordinarily cautious and conservative businessman. As CFO of Disney, his main role was to be a critical and reliable “No” man to then-CEO Michael Eisner. “You can hold back the bridle of a fast horse more easily than you can get a dead horse to move,” Eisner once told me. “I liked having Richard say, ‘No, we shouldn’t do that. Stop. It’s irresponsible.'”

Nanula was key to Disney’s success in the ’90s. When EuroDisney was sucking billions of capital, the young Nanula negotiated with Saudi Arabia’s Prince Alwaleed bin Talal bin Abdul Aziz Alsaud to help save Paris’s version of Disneyland from bankruptcy. Actually, Nanula was so cautious that he discouraged Eisner from buying a TV network—which Eisner went ahead and did, acquiring ABC in a $19 billion deal in 1995. That purchase brought Disney ESPN and turned out to be a very wise investment.

Nanula went on to head Disney Stores and was on the company’s fast track for 12 years before his best friend from Harvard Business School, Barry Sternlicht, wooed him away to run Starwood Hotels and Resorts. “I wanted to kill him,” Eisner told me about Nanula’s departure in 1998. “I was aggravated and personally hurt.” Eisner gave Nanula his blessing, however, since Starwood—Sternicht’s patched-together hotel empire, following a hard-fought takeover battle for ITT-Sheraton –was “a once-in-a-lifetime opportunity” to lead a Fortune 500 company.

At Starwood, the golden boy’s success began to unravel. Nanula, calm and self-possessed, and Sternlicht, who is intense and impetuous, couldn’t stand working with each other. They veered toward divorce corporate-style. That is the title of my 1999 Fortune story about their time at Starwood. Nanula quit the day the Fortune story came out. The two men, who had once been in each other’s weddings and had gone on family vacations and trips to the Super Bowl together, didn’t talk until they ran into each other years later.

Nanula never got back on the fast track. Dismissing Eisner’s invitation to go back to Disney, he briefly ran an Internet startup, Broadband Sports, whose investors included Michael Dell and former Viacom CEO Frank Biondi. The firm went bust a year or so after Nanula joined. His tenure as CFO of Amgen lasted six years but ended with controversy over late public disclosure of disappointing drug-study results.

At Los Angeles-based Colony Capital since 2008, Nanula was viewed as a smooth and astute businessman who operated below the radar. As chairman of Miramax, which Colony acquired from Disney in 2010, he was trying to restore the film studio to the stature it had when it was run by the prolific Weinstein brothers.

There has always been a bit of mystery to Richard Nanula. The son of an Italian father and an African-American mother, he once told me that his parents’ divorce when he was six years old was very hard on him—and his way of handling their breakup was to make things seem just fine. That’s how Nanula handled most hardships in his life. Today, Nanula has little chance to fulfill the high expectations that so many people once had of him. He didn’t respond to an email that I sent to his personal account or to a voicemail left at his home.