Rich was trained in law and in 1533 became solicitor general. He was involved in the treason trials of Sir Thomas More (More, Sir Thomas) and Bishop John Fisher in 1535, and it was specifically his testimony against More that led to More's conviction. He then helped Henry's chief minister, Thomas Cromwell, carry out the dissolution of the monasteries. In 1536 he was elected speaker of the House of Commons, and by 1540 Rich had become a privy councillor.

Shortly after Henry VIII's death in January 1547 he was made Baron Rich, and in October he became lord chancellor. Two years later he helped John Dudley, Earl of Warwick (later Duke of Northumberland (Northumberland, John Dudley, duke of, earl of Warwick, Viscount Lisle, Baron Lisle)), overthrow Edward Seymour, Duke of Somerset, regent to young Edward VI, but in 1551 Rich resigned the lord chancellorship on grounds of ill health. Like the other councillors, he acquiesced when Edward illegally designated Northumberland's daughter-in-law Lady Jane Grey successor to the crown; nevertheless, after Edward's death in 1553, Rich changed sides and supported the cause of Mary Tudor, a Roman Catholic who ruled as Queen Mary I. Thereafter, his health prevented him from taking a prominent part in public affairs.