The elder son of Sir William Cowper, 2nd Baronet, he was educated at St. Albans School and was called to the bar in 1688. Having promptly given his allegiance to William of Orange, he was made recorder of Colchester in 1694 and from 1695 to 1705 was a member of the House of Commons.

Having survived the odium of a murder charge brought against his brother Spencer (the grandfather of poet William Cowper), Cowper was appointed lord keeper of the great seal in 1705. In 1706 he succeeded to his father's baronetcy and was raised to the peerage as Baron Cowper. In 1707, after the union with Scotland, he was appointed the first lord high chancellor of Great Britain. He resigned in 1710. On the death of Queen Anne, he was made one of the lords justices for governing the country during the interregnum, and Cowper wrote a paper entitled An Impartial History of Parties for George I's guidance. He was reappointed lord chancellor in 1714, supported the impeachment of Lord Oxford in 1715, and presided, in 1716, as lord high steward at the trials of peers who had been charged with complicity in the Jacobite Fifteen Rebellion of 1715. In 1718, shortly after receiving the titles Viscount Fordwich and Earl Cowper, he resigned, probably because his intimacy with, and support of, the Prince of Wales (afterward George II) had incurred George I's enmity. Earl Cowper remained an active member of the House of Lords until his death. His eldest son, William, succeeded to the family honours.