SHANGHAI—A Chinese court handed down a 10-year prison sentence to the principal defendant in the closely watched Rio Tinto PLC trial here, a dramatic and closely watched case that abruptly swung from politically charged accusations of espionage to admissions of bribery over iron-ore contracts.

The Anglo-Australian mining company—which had once defended its four accused employees—swiftly fired them Monday, saying there was "clear evidence" they took bribes. The company described the actions of the convicted employees as "deplorable"...