In his office he is said to have made the working drawings for the erection of Stone Buildings, which are still preserved at Lincoln's Inn, and to have designed Howletts, in the parish of Bekesbourne, Kent.

On the recommendation of his old fellow-pupil, Samuel Pepys Cockerell, and other friends, Leach abandoned architecture for the law, and was admitted a student of the Middle Temple on 26 January 1785.

[1] Having studied of conveyancing and equity drafting in the chambers of William Alexander, he was called to the bar in Hilary term 1790, and joined the home circuit and Surrey sessions.

In 1800 Leach gave up all common law work, and confined himself to the equity courts, where his pleadings and terse style of speaking secured him an extensive business.

In March 1809 he defended the conduct of the Duke of York and Albany, and on 31 December 1810 supported William Lamb's amendment to the first regency resolution (ib.

[2] Early in February 1816, Leach vacated his seat in the House of Commons by accepting the Chiltern Hundreds, and was immediately afterwards appointed by the Prince Regent as Chancellor of the Duchy of Cornwall.

Resigning these posts, he succeeded Sir Thomas Plumer as vice-chancellor of England in January 1818, and having been sworn a member of the Privy Council on 30 December 1817, was knighted in the following month.

By an act of parliament passed in August 1833 Leach became, by virtue of his office as Master of the Rolls, a member of the Judicial Committee of the Privy Council.