The new position will take Niall, 18, from Par, a step closer to achieving his ambition of becoming a head gardener like his mentor, Gary Long, head gardener at Trewithen.
Niall joined the volunteer team at Trewithen within days of leaving school. He had already been accepted onto the two-day-a-week National Certificate in Horticulture (NHC) course at Duchy College, Rosewarne, and had been advised to get work experience by his college tutor who introduced him to Gary.
He completed the NHC last year, went on to study for the Advanced NHC and is currently waiting for the results of those exams.
Gary said: "He's been fantastic, not just doing his three week days with us but spending every holiday working here as well and he really does work hard. Niall reminds me of myself, he's focused, has a great attitude and he's prepared to put in the effort.
"By the time he's 20 he'll have done two years at Trewithen and two years at Caerhays and as far as experience goes I don't think you can get much better than that! We're all going to miss him but he's destined to go on to big things. I just hope we haven't seen the last of him."
Trewithen staff threw a celebratory cream tea party to thank Niall for all his help and wish him luck in his new job before he left the estate.
Niall said: "My time at Trewithen has been invaluable, everyone, especially Gary, has been really supportive. I've had a fantastic time and am extremely grateful to everyone at Trewithen for everything they've done to help me."