The interpreter had left one of the US force’s fire-bases in Uruzgan province to escort a local mechanic back to Deh Rawood town, the official said.
“The man was found dead in a coalition vehicle near the Deh Rawood Bazaar" on Sunday, spokesman Colonel Rodney Davis told reporters at Bagram Air Base, 50 kilometres north of Kabul.
An investigation is under way by Afghan police, Davis said without providing any further details.
Hundreds of Afghans are hired by US-led forces as interpreters, and they regularly accompany troops during combat operations.
Davis did not speculate who could be behind the killing, but attacks on US forces and bases are usually blamed on Taliban fighters and their al-Qaeda allies.
Loyalists to former prime minister and commander Gulbuddin Hekmatyar have also been blamed.
Hekmatyar is said to be trying to topple President Hamid Karzai's government and has been declared a wanted terrorist by the United States.
The Deh Rawood district in central Afghanistan was a well-known recruiting ground for the ousted Taliban government.
A US-led force of some 11,500 troops is currently hunting down Taliban and al-Qaeda hold-outs, mainly in the south and eastern provinces bordering Pakistan.