The verdict was reached today in a New York trial where Abid Naseer acted as his own lawyer.
Prosecutors say Naseer plotted in 2009 to bomb a shopping mall in Manchester, England.
The charges were dropped after a British court found there wasn't enough evidence, but US prosecutors later charged him in a broader conspiracy that included a failed plot to bomb the New York subway.
They said evidence showed Naseer was communicating with al-Qaida using women's names as code for explosives. The 28-year-old Naseer testified the emails contained innocent banter about his pursuit of a potential bride, not evidence of terrorism.