The 26-year-old has been in a coma since the poisoning in 2016.
His death was confirmed on Thursday by the state court in Bielefeld, which convicted the suspect in the case last year.
During his trial a psychologist said the poisoner, identified only as Klaus O. for privacy reasons came across like a researcher who was trying to see how different substances affected rabbits".
The poisoned man's parents spoke about his suffering during the trial last year, German news agency dpa reported.
A 57-year-old Klaus O. was sentenced to life in prison last March after the court found him guilty of attempted murder.
The defendant, who has appealed against the verdict, peppered co-workers' food with mercury and other substances over several years, leaving one in a coma and two others with serious kidney damage.
The case came to light after a colleague at a metal fittings company in the town of Schloss Holte-Stukenbrock, in north-western Germany, noticed a white powder on his lunch.
He was arrested in May 2018 after surveillance video showed him putting a suspicious powder on a colleague's sandwich at their workplace.
Following his trial in March last year, a judge ruled Klaus O would not have his sentence reduced because he was a "danger to the general public".
Tests identified it as lead acetate and mercury, almost tasteless substances that if ingested could lead to serious organ damage.
Exposure to mercury can cause a number of health issues including skin, neurological and kidney problems.
Most exposure comes from eating fish, coal and gold mining, or dental fillings.
Credit: Daily Mirror