Gary Shawkey, 47, was found guilty of special circumstances murder for financial gain. The jury also found Shawkey guilty of grand theft.
The prosecution said Shawkey took more than $1 million from 71-year-old Robert Vendrick before killing him in 2008. His body was never found.
Shawkey claimed that he believed Vendrick was living somewhere in Mexico.
Vendrick was a retired man from Phoenix who had worked in the software industry. Prosecutors say Vendrick met Shawkey online several years ago. Shawkey corresponded with him via email until finally, Vendrick trusted him with hundreds of thousands of dollars to put toward a software development plan.
Prosecutors say there was never any software development plan in the works.
Shawkey faces a maximum of life in prison without the possibility of parole at his sentencing next month.