Ramesh Balwani, a Silicon Valley executive, has been found guilty in a scheme with Elizabeth Holmes to deceive investors.
He claimed repeatedly and falsely that Theranos had invented a device that could detect hundreds if diseases from just a few drops.
Holmes was also found guilty in January at a separate trial. She had accused Balwani, who denies the allegations.
Judges will sentence Holmes, 38, as well as Balwani (57), in the autumn.
Holmes faces around 20 years imprisonment and is currently out on bail. Balwani faces up to 20 year imprisonment and millions of dollars in restitution payments for his victims.
Theranos was once valued at $9bn (PS7.5bn), however, it was founded on blood testing technology that failed to work.
On the fifth day, the verdict was delivered at the San Jose courthouse, Silicon Valley. Balwani was found guilty on all 12 charges.
During closing arguments, the prosecutors displayed text messages that Balwani had sent to Holmes during 21 June.
He wrote, 2015, "I am responsible to everything at Theranos." He wrote in 2015, "All were my decisions."
Jeffrey Schenk, Assistant US Attorney, called the message an admission to guilt. He said, "He's admitting his part in the fraud."
Sunny Balwani was Theranos number two.
Holmes was the founder and chief executive of the company. Balwani managed the day-to-day operations of the company.
Some thought that his lack of medical training might have been enough to convince a jury that Theranos' technology was not working.
His lawyers also claimed that he had invested his own money into the company. Why would he do that if the technology is not working?
However, this verdict makes Holmes just as guilty, and in some ways more culpable for the Theronos scam than Holmes.
Holmes was convicted on four counts of fraud and Balwani on twelve. Holmes did not know that he was also guilty of defrauding patients.
This means that Balwani's role in Theranos' saga will be remembered as the most important.
Silicon Valley will see executives at tech companies and start-ups.
They will also be surprised to learn that the fraud charges against an executive were broader than those against the founder or chief executive.
This is a clear legal warning for managers: it's not always their boss who carries the greatest responsibility.
Elizabeth Holmes' legal team was partially vindicated by the verdict. They placed Elizabeth Holmes on the witness stand for cross-examination. It was a risky move, but it may have paid off.
Theranos was once the darling biotech and Silicon Valley.
Holmes raised more than $900m in funding from investors including billionaires such as Rupert Murdoch, tech mogul Larry Ellison and media mogul Rupert Murdoch.
With a test that detects conditions like cancer and diabetes in just a few drops, the company promised to revolutionize healthcare.
These claims were exposed by the Wall Street Journal in 2015. They claimed that their core blood-testing technology was not working.
Balwani's three-month-old case bore many similarities to Holmes's prosecution.
Balwani's lawyers argued that Holmes also duped him after he joined the company in 2010 and became chief financial officer.
The couple took great care to keep investors and employees in the dark about their relationship before the company collapsed in 2016.
His lawyers pointed out that despite having invested $15 million of his own money in Theranos, his stock options were never cuffed despite the fact that his investment grew to $500m.
Balwani, unlike Holmes, did not stand to defend himself.
Holmes, then 18, met Balwani on a college visit to Beijing.
He was a 37 year-old South Asian immigrant from South Asia who had made millions in selling his software company prior to the dot-com bubble burst. He also divorced his Japanese wife around the time he met Holmes. They fell in love when Holmes started Theranos.
Holmes was the face and head of the company. However, Holmes was also the chief operating officer and president.
Robert Leach, the prosecutor, stated earlier that Ms. Holmes and Mr Balwani were "partners in practically everything".
Holmes and the defendant knew that the lies they told investors were false.
