Charles Foster was found guilty of assault and faces up to three years in prison when he is sentenced next month.
The jury deadlocked over similar charges against two other deputies, Michael Phelps and Nick Downey, involved in the same incident. They are expected to face a new trial in May.
"The jury has spoken, and I have the highest respect for our criminal justice system," Sheriff John McMahon said in a statement released by the department. "Let's not discourage the daily efforts of all the men and women of the sheriff's department. Our employees will continue to provide dedicated service and will always be committed to serving the public."