Mahmood and Dart were both given extended sentences and will serve two-thirds of their prison terms rather than half, and will spend five years on licence.
Dart refused to stand when he was sentenced, saying: "I don't wish to stand up, I believe ruling and judging is only for Allah."
The judge said that they were all "committed fundamentalists" who would have been prepared to kill.
He told Dart and Mahmood:
Three British Islamic extremists including a white Muslim convert and a former police community support officer have been jailed for terrorism.
Richard Dart, Jahangir Alom and Imran Mahmood, were sentenced at the Old Bailey for engaging in conduct in preparation of acts of terrorism.
Dart was jailed for six years, Alom for four years and six months and Mahmood for nine years and nine months.
Mr Justice Simon told the trio they held "radical Islamist beliefs and have shown yourselves to be committed to acts of terrorism".
They admitted the offence between July 2010 and July last year at a previous hearing last month.
A body, believed to be that of a sailor from Kingston who went missing within hours of buying a new yacht, has been found floating off the Isle of Wight.
It's believed Hany Mustapha had been attempting to sail from Dorset to Hayling Island in Hampshire, when he fell overboard.