Case, co-founder of AOL, appeared on CNBC's "Squawk Box," a day after a federal judge cleared the way for AT&T's $85.4 billion bid to buy Time Warner. The decision is expected to prompt more consolidation in media, and perhaps spark a bidding war between Comcast and Disney for assets being sold by Twenty-First Century Fox.
Reflecting on the AOL-Time Warner combination nearly 20 years later, Case said, "It really was to capture the opportunity of convergence in technology."
Just 15 years after starting America Online, he negotiated the around $160 billion mega-deal to bring together new media AOL and old media Time Warner.
However, the two companies "failed to capitalize" on that moment and execute their goals, said Case, founder of venture capital firm Revolution.
AOL and Time Warner had issues with "culture" and too much "short-term orientation," he said, adding there were people in the combined company that were not as enthusiastic about its digital path.
"They tended to play defense, trying to protect what already existed as opposed to playing offense and try to create what the future would be," he said.
AOL and Time Warner split into two separate companies in December 2009.
Case warned AT&T against suffering the same fate, though he did contend "AT&T and Time Warner and Fox and Comcast and others ... [are] really playing defense" already.
On CNBC in February, AT&T chief contended the Time Warner deal would help level the playing field against those tech powerhouses, which are both content distributors and content creators.
AT&T has long-maintained that buying Time Warner is what's called a vertical merger, meaning AT&T's mobile network, satellite TV service DirecTV and high-speed internet solutions don't directly compete with any of Time Warner's content businesses, which include HBO, CNN and the Warner Bros. studio.
Shortly after the deal announcement in October 2016, Time Warner CEO Jeff Bewkes told CNBC that the combination won't be anything like the AOL-Time Warner deal, which is considered one of the worst mergers of all time.
For its part, AT&T said at the time that it wanted to buy Time Warner to diversify its revenue, and also become a media powerhouse that could attract consumers by bundling entertainment with mobile service.
Disclosure: Comcast is the owner of NBCUniversal, parent company of CNBC and CNBC.com.