"He comes up with a good idea, goes to the chairman, and says this is what I think. He is innovative - great at a time when bankers run around after the same stuff, most of them coming up with cookie-cutter deals," said one person who used to work with him, asking not to be named.
Hannam is behind a string of deals in mining that has transformed Britain's blue-chip share index, but has also left his mark on the telecoms, media, financials and household products sectors thanks to a 28-year career that has taken him to some of the world's most prestigious banks.
That reputation is now at risk.
Britain's Financial Services Authority on Tuesday fined Hannam 450,000 pounds for market abuse, a decision the 56-year-old Briton is appealing against, though he has resigned from JPMorgan to focus on challenging the regulator.
Raised and schooled in Bermondsey, a tough area of south London, Hannam is not a typical senior JPMorgan banker.
His banking career began in 1984 when he joined Salomon Brothers. He progressed through British merchant bank Robert Fleming, which was bought by JPMorgan in 2000.
He then teamed up with Cazenove in a joint venture in 2004, before the U.S. investment bank fully bought out Cazenove in 2010.
Hannam was angered after being passed over for the top job at Cazenove in 2008, and one report said he sulked for two weeks in New Zealand, turning off his phone.
But he stayed on as one of the U.S. bank's top rainmakers, in the role of chairman of JPMorgan Capital Markets.
His biggest deals include the merger that created BHP Billiton and its subsequent listing to create a $180 billion mining giant. He has advised on a string of deals for Xstrata , helping to transform it into a commodities powerhouse.
"Ian has been a trusted adviser of mine for many years including advising Xstrata from its inception. He has always acted with honesty and integrity and I have greatly valued his counsel," Xstrata Chief Executive Mick Davis said in a statement to Reuters.
NO ONE LIKES US
Married, with three children, Hannam lives in Notting Hill, an expensive London neighbourhood, but still supports Millwall Football Club, a second-tier soccer team based where he grew up.
Millwall are renowned for their chant "No one likes us, we don't care", and people who have worked with Hannam said he can prompt a love/hate relationship. Extremely hard-working, he dislikes fools and likes others to match that ethic.
At age 17, Hannam passed the service's tough selection process and joined the Artists Rifles, a part-time volunteer regiment.
While serving, he studied for a degree in civil engineering from London's Imperial College, graduating in 1977.
After a spell with construction firm Taylor Woodrow, helping build roads and other infrastructure in Oman and Nigeria, he went to study at the London Business School and in 1984 joined the Salomon Brothers training programme in New York.
One report said he arrived there by parachuting into the United States with a Special Air Service (SAS) unit that was training with American special forces, and then made his way to New York.
Almost 30 years on, he is renowned for spotting "undervalued" opportunities in emerging markets, often bringing them to market in London through an initial public offering or by doing deals like the Rothschild reverse takeovers.
FTSE 100 firms he has brought to market include Vedanta , Kazakhmys , Fresnillo and African Barrick Gold (ABG) . He advised Wellcome Trust on one of the biggest share sales of the early 1990s, and HSBC on its $18 billion rights issue three years ago.
The listing of ABG showed how he works. He visited Barrick Gold's top brass with the idea that its African businesses were not properly valued in its current structure, and they should be separated and listed in London, where investors would put more value on the assets. The firm took his advice, and ABG is now worth $2.54 billion.
Working in mining has taken Hannam to many danger zones, and he has been active in recent years in Afghanistan.
"You need a madman or a genius - I am not sure which one he is - but maybe you need a madman and a genius to get these things done," said one person who has worked with him.
He regarded Afghanistan as a resource-rich, untapped opportunity that could supply China and India with iron, copper, rare earth metals, and gold.
A Fortune magazine report last year said that had left a lot of powerful people, including U.S. General David Petraeus, as counting on him to demonstrate that the country is safe for foreign investors.
In a profile on his efforts there, Fortune said: "If anyone can wrest a fortune from Afghanistan's rubble, it is this man, Ian Hannam."
(Additional reporting by Sarah White and Victoria Howley; Editing by Giles Elgood)