The prosecutor called him a "thief in government house".
Ibori, once seen as one of Nigeria's wealthiest and most influential politicians, was arrested in 2010 in Dubai and then extradited to London.
Some $35m of his alleged UK assets were frozen in 2007.
As his trial at London's Southwark Crown Court was about to begin, Mr Ibori changed his plea to guilty and admitted stealing money from Delta state and laundering it in London through a number of offshore companies.
Paul Whatmore Metropolitan PoliceWe will now be actively seeking the confiscation of all of his stolen assets so they can be repatriated for the benefit of the people of Delta state”
"He was never the legitimate governor and there was effectively a thief in government house. As the pretender of that public office, he was able to plunder Delta state's wealth and hand out patronage."
He is due to be sentenced on 16 April.
'Lavish lifestyle' Ibori's wife, Theresa, his sister, Christine, his mistress, Udoamaaka Okoronkwo, and his London solicitor, Bhadresh Gohil, have all been convicted of money-laundering.
Their convictions could only be reported on Monday after reporting restrictions were lifted.
"We will now be actively seeking the confiscation of all of his stolen assets so they can be repatriated for the benefit of the people of Delta State."
He said that the money Ibori stole should have been used to pay for sanitation, power supplies and healthcare for some of the poorest people in the world.
Ibori spent some of the stolen money buying six houses in London - he paid £2.2m in cash for one Hampstead mansion - and put his children in expensive British private schools, police say.
DIY theft Three of the money-laundering charges relate to the sale of Delta state's share in mobile phone company V-Mobile to neighbouring Akwa Ibom state.
This raised $37m which Ibori stole and laundered through his solicitor Gohil, who has been jailed for 10 years.
In 2007, a UK court froze assets allegedly belonging to him worth $35m.
His annual salary as Delta state governor was less than $25,000.
He had already left the UK when his assets were seized but he was arrested in Nigeria later that year.
The charges were dismissed but he was then re-arrested in Dubai on a British warrant.
Ibori has been convicted in the UK before - in 1991 for stealing, in cahoots with his wife who worked as a cashier, from the Neasden, London, branch of the DIY shop Wickes.
He was also found guilty in 1992 for being in possession of a stolen credit card and having used it to fraudulently withdraw $1,590.
Nigeria is one of the world's biggest oil producers. Under its federal system, state governors enjoy huge powers and control budgets larger than those of many West African countries.
They have immunity from prosecution while in office.
Two other former state governors have been convicted of corruption charges in Nigeria, after fleeing the UK while free on bail.
The rise and fall of James Ibori
- 1958: Born in Delta state, UK police believe, saying he previously lied about his age
- 1980s: Moved to UK
- 1991: Convicted of stealing from DIY shop Wickes
- 1992: Convicted of credit card fraud
- 1993-4: Allies himself to Nigeria's then military ruler Sani Abacha
- 1999: Elected Delta state governor
- 2007: Stepped down as governor
- 2007: UK assets worth $35m frozen
- December 2007: Arrested in Nigeria on corruption charges
- 2009: Nigeria court dismisses charges
- April 2010: Ibori's supporters attack police as they try to arrest him
- May 2010: Arrested in Dubai
- 2011: Extradited to UK
- 2012: Pleads guilty to money-laundering in London court