Smith's decision, announced Thursday with the release of quarterly financial results, comes only four months after the Ottawa-based company went public.
However, in an interview he said that in the months leading up to the initial public offering he wasn't considering an exit.
"Now seemed as good a time as any," he said. "We'd achieved as many of the things I had as personal objectives since I came back to Mitel almost a decade ago." In addition, the 62-year-old Smith noted he's been working 40 years.
His departure comes after leading Mitel through what could be considered a complete overhaul: Shifting to a software-based company from a PBX manufacturer, buying U.S. competitor Inter-Tel Inc. in 2007 and going public in April.
As a result, he says research companies rate Mitel the third biggest IP communications supplier in the U.S. either by ports shipped or revenue to small and mid-sized businesses (SMBs).