The executive was more focused on SpaceX until things "melted down" and he became CEO of Tesla in 2008.
Elon Musk has admitted his biggest career mistake traces back to Tesla's early days when he was spending more time focused on SpaceX.
Speaking at a Q&A panel for SXSW, the executive says he took for granted Tesla's success when serving as chairman from 2004-2008 until "things really melted down" and the company faced looming bankruptcy amid a broader industry collapse.
"I thought, 'I can keep running SpaceX. I'll dedicate 20 percent of my time to Tesla, and that'll be fine," he said of his time as chairman before taking the reins as CEO in October 2008.
"Whenever you think you can have your cake and eat it too, you're probably wrong," he added.
Tesla is on a much stronger footing now than in 2008, however Musk warned SXSW attendees that electric vehicles and rockets make for some of the "dumbest" investments in terms of risk-adjusted rate of returns.