Stoddard was previously general manager for digital health at Comcast Corp. His appointment is effective Tuesday, a spokeswoman for the venture said on Tuesday.
Amazon, Berkshire and JPMorgan announced the joint venture in January, saying U.S. healthcare costs were rising too fast and holding back economic growth.
The venture is being led by well-regarded surgeon, author and industry critic Atul Gawande, who was appointed as chief executive officer in June.
The joint venture will use big-data analysis and other high-tech tools to improve care and cut costs.