Taysir Hayb, a former private, was found guilty of shooting 22-year-old Tom Hurndall - a member of the International Solidarity Movement that has frequently criticised Israeli occupation force activities in the Palestinian territories.
Hurndall died in a London hospital in January 2004 after having lain in a coma for nine months.
He was shot while helping Palestinian children cross a street to avoid Israeli gunfire in the southern Gaza town of Rafah, which Israel has occupied since the 1967 Middle East war.
A three-judge Israeli military tribunal sitting in the southern Israeli town of Kastina also convicted Hayb of obstruction of justice, giving false testimony and soliciting false testimony.
Sentencing was set for a later date, but manslaughter carries a maximum 20-year jail term under Israeli military law. British police have launched a separate investigation into the shooting.
Meanwhile, Hurndall's mother, Jocelyn Hurndall, told BBC Radio that Israeli authorities denied entry to two of her deceased son's brothers who had wanted to attend Hayb's trial.
"They were detained at the airport," she said. "We do find it wholly unacceptable."
Israeli Foreign Ministry spokesman Mark Regev said the government had discussed such a visit with British officials.
Tom Hurndall was in a coma for
nine months before dying
"Unfortunately it appeared the purpose of the visit wasn't strictly innocent," he said. "As a result the family chose not to take up our offer for them to attend the legal proceedings."
An Israeli official who asked not be named said Hurndall's brothers had declined to sign a guarantee they would not take part in pro-Palestinian demonstrations while in the country.
Few crimes investigated
Human rights groups maintain that Israel has used excessive force to quell Palestinian dissatisfaction with the occupation.
Last week, Human Rights Watch accused the army of investigating fewer than 5% of hundreds of cases of Palestinian civilians killed since 2000.
But the army says it does its utmost to avoid civilian casualties during clashes with Palestinian resistance groups and that it investigates fatalities when criminal wrongdoing is suspected.
Last month, the army sentenced a soldier to 18 months in prison for shooting and wounding an unarmed Palestinian man in Gaza as he fixed an antenna on his roof in Gaza.