On CNBC's "Fast Money," the founder of Andreessen Horowitz differentiated between forms of activist investors.
"Activism is one of those terms like 'hedge fund.' It depends what you mean," he said. "So, activism in the form of Ralph Witworth is fantastic. I can't say enough good things about Ralph. He's astonishing. He's a teacher, he's a mentor."
Activist investor Witworth was appointed to the board of Hewlett-Packard in November 2011 and helped steady the company.
Icahn is a different story, Andreessen said.
"Carl's like literally the exact opposite," he added. "Remember the 'Star Trek' episode where they've got the good Captain Kirk and the evil Captain Kirk. … Carl's like the exact opposite. … He lies, he just makes stuff up. He slanders. … It's like his inner 6-year-old comes out."
In the 1966 "Star Trek" episode, "The Enemy Within," a transponder malfunction beams up two versions of Captain James Kirk, one of which conspires against the USS Enterprise crew.
Andreessen said that Icahn cared little for the companies in which he becomes involved. "He's not in the details of any of this stuff."
Icahn, an investor in eBay, had been pushing for the company to spin off its electronic-payment unit, PayPal, into a separate company. Last month, eBay announced it would do just that in 2015.
Andreesen, who sits on eBay's board of directors, also downplayed whether Icahn was right.
"I don't know that it takes some super high IQ to say that it might be a good idea to break companies up under certain circumstances," he said. "I think it's a question of which companies, when, with which strategies."
Andreessen also talked about the human impact.
"By the way, you're affecting the lives of real people. You're affecting the lives of tens of thousands of people. You're affecting the destiny of these companies for years to come. The substance on this really, really matters," he said. "And all the crazy bomb-throwing and all the inflammatory statements and all that stuff is just a complete sideshow. And, like, if that's how he wants to do business, he can. But it doesn't actually contribute to building a company."
Icahn could not immediately be reached for comment.