Mr. Hamilton was by and large replaced by Tim Jones, who was promoted from chief strategy officer, when Mr. Hamilton announced his leave.
Mr. Jones circulated a memo this morning announcing the news.
It contained a quote from Mr. Hamilton that read: "It's time for me to move on to the next project. The best thing about the past 8 years has been working with all of you. We did great stuff together. Thank you for who you are and all you do. Onward!"
Zenith declined further comment. Mr. Hamilton could not immediately be reached for comment.