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.
White Muslim convert Richard Dart, 30, was jailed for six years at the Old Bailey today for terrorist offences. He refused to stand as he was sentenced.
- Police recovered fragments of text from Richard Dart's laptop which referred to Wootton Bassett
- Dart changed his name to Salahuddin al-Britani and became involved in extremism after moving from his home town Weymouth in Dorset to east London
- His beliefs were brought into the spotlight as part of a television documentary "My Brother the Islamist" made by his step brother Robb Leech
- In the film he was seen preaching in the street, attending extremist lectures
Thousands of people have taken part in a motorbike ride through Royal Wootton Bassett to pay their respects to the British service men and women killed in Afghanistan. They also thanked the town for their support of the families of soldiers killed in action.