Diamond joined Barclays on July 4, 1996,[17] and in September 1997 became a member of the executive committee of the company, Britain's second largest banking group.

[21][22] On 30 June 2006, The Wall Street Journal ran a front page, column one, article detailing how Barclays Capital was making a giant portion of its income not through legitimate investment banking activities but through a tax dodge, a so-called "double dip", which was essentially paid for by the British and American taxpayer.

[20] Diamond then sealed an agreement with Lehman Brothers President and COO Bart McDade to purchase key assets of that firm after it filed for bankruptcy, which instantly gave Barclays an investment banking foothold on Wall Street.

[6] Diamond's longtime protege, Jerry del Missier, who had been appointed chief operating officer of Barclays on June 22, 2012, resigned on July 2, 2012.

[29] The "Double Dip" tax scams were fairly simple - in a typical case, Barclays and an American bank would loan, say, an airline for the purchase of a jumbo jet.

This practice ended after The Wall Street Journal published an exposé of the dodge in a front page, column one, article on 30 June 2006 by Carrick Mollenkamp, which ensured that Parliament, the Bank of England, and the UK Inland Revenue and the American Internal Revenue Service would see it and become aware of the scam, and the practice was subsequently outlawed, thus eliminating a major source of income for Barclays.

[33] The FSA's director of enforcement described such behaviour as "completely unacceptable", adding "Libor is an incredibly important benchmark reference rate, and it is relied on for many, many hundreds of thousands of contracts all over the world.

However, following widespread anger at his refusal to step down and amidst concerns that his presence could be harmful to the Barclays brand, he resigned as chief executive on July 3, 2012.

[35] According to an article in The New York Times published July 16, 2012, a former senior Barclays executive claimed he had received instructions from Robert Diamond to lower Libor rates after Diamond's discussions with Paul Tucker, deputy governor of the Bank of England, in which they had discussed the bank's financial position at the height of the 2008 financial crisis.

[36] Diamond, who reported that he was "sickened" by news of the Libor scandal, was surprised to see calls for his departure since Barclays had taken the advice of its lawyers to cooperate in the investigation and be first bank to settle the charges.

The New York Times noted that Diamond's role in the scandal was minimal and suggested that the real reason for his sacking was that he had become the "unacceptable face of banking".

[38] On November 28, 2013, Diamond and Rwandan-based entrepreneur Ashish Thakkar founded Atlas Mara, a company whose focus is banking in the African continent.

[44] In 2011 he was included in the 50 Most Influential ranking of Bloomberg Markets Magazine before his ethics problems appeared and the Bank of England terminated his employment at Barclays.