Exeter Magistrates' Court was told that the owner of Ed Rogers Landscape Construction, Edward Rogers, had shown the teenager how to operate the log splitter and then left him to work alone on the site.
On the day of the accident, he held a log in place and started the machine. The blade came down on the piece of wood and severed his finger.
He was not given any first aid, the Health and Safety Executive (HSE) found, and when he was later taken to hospital, his finger could not be saved due to the severity of the injury.
Ed Rogers Landscape Construction and Maintenance pleaded guilty to breaching s 2(1) of the Health and Safety at Work Act. It was fined £6,000 plus costs of £1,017 on 28 February.
HSE inspector Simon Jones said: "The boy being isolated made the situation worse when there was no one on site to help him after suffering this appalling, life-changing injury."