A former banker and Labour government adviser he had defected to the Conservative Party in late 2008. Until 2010 he acted as an advisor to the Shadow Chancellor, was a member of the Economic Recovery Committee of the Shadow Cabinet and conducted The Tripartite Review. He joined the House of Lords in 2010. 
Defection to the Tories
The Daily Telegraph reported Sassoon's defection from Labour in 2008 as follows:
- Sir James Sassoon is to be unveiled as a Tory supporter only weeks after quitting as an adviser to the Prime Minister. Earlier this month it was announced that Sir James had relinquished his position as the Treasury's Special Representative for Promotion of the City. His departure led to charges that Labour's relations with the Square Mile had hit a new low following earlier rows over issues including changes to capital gains tax and a tax crackdown on "non-domiciled" foreign citizens in the UK. Sir James has now agreed to become an adviser to George Osborne, the Conservative shadow chancellor on the reform of the regulatory regime covering the banking sector.
Business 'buddies'UKTI's Lord Green, Sassoon was designated 'business buddy' to six big energy and nuclear companies, Eon, EDF, Iberdrola, Centrica, RWE and Scottish & Southern Energy (SSE).  
Dining with lobbyists
In April 2011 Sassoon dined with an unnamed representative from lobbying firm Edelman.
From HM Treasury's website as of 2012:
- James Sassoon spent most of his career in the City, starting at KPMG (Thomson McLintock & Co) in 1977. From 1985 he was at SG Warburg (latterly UBS Warburg), leading the firm’s global privatisation business and ending as Vice-Chairman, Investment Banking.
- In 2002, he joined HM Treasury as Managing Director, Finance, Regulation and Industry. In this position, he also chaired the Standing Committee on Financial Stability, was a non-executive director of Partnerships UK and sat on the global Financial Stability Forum and on the EU Financial Services Committee. From 2006 to 2008 Sir James was the Chancellor’s Representative for Promotion of the City and in 2007/08 served as President of the Financial Action Task Force (the global anti-money laundering and counter-terrorist financing organisation).
- In 2008-10, he acted as an adviser to the Shadow Chancellor, was a member of the Economic Recovery Committee of the Shadow Cabinet and conducted The Tripartite Review.
- Other positions held include chairman of The Merchants Trust plc 2010 (director 2006-10), chairman of the ifs School of Finance 2009-10, director of the Nuclear Liabilities Fund 2008-10, trustee of The British Museum 2009-10 and of the National Gallery Trust 2002-09.
- Born in 1955; educated at Eton and Oxford; FCA; appointed Knight Bachelor 2007; married with three children. 
In 2008 his biography stated:
- [Sassoon is] the UK Chancellor's and the UK Treasury's Representative for Promotion of the City, a part-time role in which he will continue. He was appointed to this role in December 2005 to champion the interests of the UK's financial and business services firms and markets internationally.
- From 2002 he served three years as a member of the Treasury Board and Managing Director of the Treasury's Finance and Industry Directorate. The Directorate was responsible for financial services and for industry policy, including productivity, enterprise and growth and public-private partnerships. He was the Chancellor's Deputy on and chaired the UK Standing Committee on Financial Stability, the Treasury's non-executive director on the board of Partnerships UK and representative on a number of Ministerially chaired committees, the global Financial Stability Forum and the EU Financial Services Committee.
- From 1985 - 2002 Sir James Sassoon was at UBS Warburg, latterly as Vice Chairman, Investment Banking with responsibility for the firm's global privatisation business. He advised Government departments on a variety of public-private sector projects as well as acting for governments in Europe, Asia and the Americas over more than ten years. He qualified as a chartered accountant with KPMG.
- Sir James was awarded a knighthood in recognition for his services to the finance industry and to public service in the Queen's 2008 New Year Honours list.
Commercial Secretary role
Sassoon's role as Commercial Secretary covered:
- Treasury spokesman in the House of Lords.
- Enterprise and productivity, including industrial strategy, better regulation, competition policy, infrastructure, and microeconomic reform.
- Corporate finance, including public corporations and public private partnerships.
- Assisting the Financial Secretary on financial services and banking policy, including UK Financial Investments (UKFI).
- Working with the Financial Secretary to promote the Government’s financial services policies and the competitiveness of the UK.
- Asset freezing and financial crime.
- Foreign exchange reserves and debt management policy, National Savings and Investment and Debt Management Office.
- Member, Global Advisory Board, Mitsubishi UFJ Financial Group
- Jardine Matheson Holdings (diversified holding company with mainly Asian interests)
- Matheson & Co (London subsidiary of Jardine Matheson Holdings)
- Hongkong Land Holdings (Asian property; director's fees ceded to Jardine Matheson and not personally received by Lord Sassoon)
- Dairy Farm International Holdings (Asian food retailing; director's fees ceded to Jardine Matheson and not personally received by Lord Sassoon)
- Mandarin Oriental International (hotels; director's fees ceded to Jardine Matheson and not personally received by Lord Sassoon)
- Jardine Lloyd Thompson (insurance broking; director's fees ceded to Jardine Matheson and not personally received by Lord Sassoon)
Abbott Laboratories (healthcare) | ANZ Banking Group Ltd (banking) | Argo Investments Ltd (Australian investment company) | BAE Systems plc (aerospace and defence) | Bank of Montreal (banking) | Bank of Nova Scotia (banking) | BCE Inc (communications) | BHP Billiton (mining) | Brambles Ltd (pallets and logistics) | British American Tobacco plc (tobacco) | British Assets Trust plc (global investment company) | Canadian Imperial Bank of Commerce (banking) | Chevron Corp (oil and gas) | ConocoPhillips (oil and gas) | Devon Energy (oil and gas) | Diebold Inc (security systems) | Dunedin Income Growth Investment Trust (UK investment company) | Emerson Electric Co (industrial manufacturing and technology) | Experian plc (support services) | GlaxoSmithKline plc (pharmaceutical) | Honeywell International Inc (technology manufacturing) | International Business Machines Corp (information technology) | IMI plc (industrial engineering) | Ingersoll-Rand plc (engineering and manufacturing) | Johnson & Johnson(medical and consumer products) | Kimberly-Clark Corp (personal and healthcare products) | Marston's plc (brewing and pub retailing) | Merchants Trust plc (UK investment company) | Morgan Advanced Materials plc (advanced materials) | Raytheon Co (defence and aerospace) | Rio Tinto plc (mining) | Royal Dutch Shell plc (oil and gas) | Standard Chartered plc (banking) | Unilever plc (food and household products) | Xylem Inc (water technology) | Jardine Matheson Holdings (diversified Asian interests) | Allegion plc (security products) | Murray Income Trust (UK investment company) | City of London Investment Trust plc (UK investment trust) | Inmarsat plc (satellite communications)
- Financial Action Task Force
- Director of Hongkong Land, Dairy Farm and Mandarin Oriental.
- Sarah Jenkins, now a lobbyist at Fleishman-Hilliard was his former private secretary at HM Treasury from 2009 – April 2012.
- Speech by the Commercial Secretary to the Treasury, Lord Sassoon; Summer Lecture to the British Bankers’ Association, 20 June 2012
- Commercial Secretary to the Treasury: Lord Sassoon, HM Treasury website, accessed 5 October 2011
- James Kirkup, Gordon Brown's former City envoy James Sassoon defects to Conservatives Daily Telegraph, Last Updated: 5:28PM BST 28 Sep 2008
- Elizabeth Rigby, Lord Green’s matchmaking finally revealed, FT Blogs, 2 November 2011, acc 29 October 2013
- HM TREASURY QUARTERLY INFORMATION 1 APRIL – 30 JUNE 2011, Commercial Secretary, 26 April 2011 Edelman, Dinner
- HM Treasury 1 October - 31 December 2010 Gov.uk, accessed 13 October 2014
- APPC Register Entry for 1 December 2011 to 29 February 2012, accessed 19 May 2012, and APPC Register Entry for 1 Mar 2012 to 31 May 2012
- APPC Register Entry for 1 Sep 2011 to 30 Nov 2011
- HM Treasury Commercial Secretary to the Treasury: Lord Sassoon, accessed 28 May 2012
- Financial Action Task Force Sir James Sassoon, accessed 15 March 2009
- Lord Sassoon Parliament.UK, accessed 23 December 2014