SAN JOSE, Calif.—A federal jury convicted Ramesh “Sunny” Balwani, the former top lieutenant to Theranos Inc. founder Elizabeth Holmes, on all 12 charges that he helped perpetuate a yearslong fraud scheme at the blood-testing startup. The verdict is the second conviction against Theranos leadership and comes six months after a jury found Ms. Holmes guilty of fraud; it secures another major victory for the U.S. government, which brought the case against the pair in 2018. It brings to conclusion one of Silicon Valley’s most notorious startup implosions, which saw nearly $1 billion of investor money evaporate after revelations that the company delivered inaccurate blood-test results to patients, including for life-threatening conditions, and Ms. Holmes and Mr. Balwani lied about its proprietary technology.