Mr MacGregor, 57, who asked to leave the Government in July, worked at the bank for 11 years before his 15-year ministerial career.
Hill Samuel was selected to advise on the rail link in August last year after a competition among a number of banks. Mr MacGregor asked to take no part in the selection process because of his links with the bank.
Now he has been appointed non-executive deputy chairman of Hill Samuel, part of TSB Group. No details of his pay have been released, but the job is bound to be well rewarded. Mr MacGregor is still MP for South Norfolk.
His appointment comes after similar jobs for other ex-ministers, such as the former Chancellor Lord Lawson - who joined Barclays - and Francis Maude, who was Financial Secretary to the Treasury and went to the US investment bank Morgan Stanley.
Before he was appointed to the Department of Transport in 1992, Mr MacGregor served as Chief Secretary to the Treasury, Minister of Agriculture, Secretary of State for Education, Lord President of the Council and Leader of the Commons.
Hill Samuel's chief executive, Hugh Freedberg, said Mr MacGregor's extensive experience would be valuable in continuing to develop the business in Britain and internationally.
Mr MacGregor joined Hill Samuel in 1968 and worked as personal assistant to the then chief executive, Kenneth Keith.
He later became a director of the bank with responsibility for business development until he joined the Government.Reuse content