The son of top Sydney silk John Agius has escaped a jail sentence for the armed robbery of a Mosman jewellery store because he was in the grip of mental illness.

Felim Agius, 20, walked into the Eskae Jewellery boutique just after 5pm on April 15 last year and threatened owner Sam Kritsotakis with a replica Beretta M92 9mm semi-automatic pistol, before hitting him on the head. He pleaded guilty to assault with intent to rob armed with an offensive weapon in the local court five months later and faced a maximum of 25 years in prison.

Suspended sentence: Felim Agius with his father John. Credit:Edwina Pickles

In the Downing District Court on Friday, Felim was represented by another leading Sydney barrister, the immediate past president of the NSW Bar Association, Phillip Boulten, SC.

Mr Boulten said Felim had a "troubled upbringing despite the benefit of having the parents that he had", partly because of the acrimonious breakdown of his parent's marriage when he was young.