His exceptional legal abilities were rewarded early with official promotion. On November 16 1616 he was made recorder of London in spite of Francis Bacon's opposition, who, although allowing him to be "a well trained and an honest man," objected that he was "bred by my Lord Coke and seasoned in his ways." On March 14 1617 he was appointed Solicitor General and was knighted.
On November 1 1625 he was made lord keeper of the great seal; in this capacity he delivered the Charles I's reprimand to the Commons on March 9 1626, when he declared that "liberty of counsel" alone belonged to them and not "liberty of control." On April 10 1628 he received the title of Baron Coventry of Aylesborough in Worcestershire. At the opening of parliament in 1628 he threatened that the king would use his prerogative if further thwarted in the matter of supplies. In the subsequent debates, however, while strongly supporting the king's prerogative against the claims of the parliament to executive power, he favoured a policy of moderation and compromise. He defended the right of the council in special circumstances to commit people to prison without showing cause, and to issue general warrants. He disapproved of the king's sudden dissolution of parliament, and agreed to the liberation on bail of the seven imprisoned members on condition of their giving security for their good behaviour.
He showed less subservience than Bacon to the Duke of Buckingham, and his resistance to the latter's pretensions to the office of Lord High Constable greatly incensed the duke. Buckingham taunted Coventry with having gained his place by his favour; Coventry replied, "Did I conceive I had my place by your favour, I would presently unmake myself by returning the seal to his Majesty." After this defiance Buckingham's sudden death alone probably prevented Coventry's displacement.
He passed sentence of death on Lord Audley in 1631, drafted and enforced the proclamation of June 20 1632 ordering the country gentlemen to leave London, and in 1634 joined in William Laud's attack on the Earl of Portland for peculation. The same year, in an address to the judges, he supported the proposed levy of ship money on the inland as well as the maritime counties on the plea of the necessity of effectually arming, "so that they might not be enforced to fight," "the wooden walls" being in his opinion "the best walls of this kingdom."
In the Star Chamber Coventry was one of John Lilburne's judges in 1637, but he generally showed conspicuous moderation, inclining to leniency in the cases of Richard Chambers in 1629 for seditious speeches, and of Henry Sherfield in 1632 for breaking painted glass in a church. He prevented also the hanging of men for resistance to impressment, and pointed out its illegality, since the men were not subject to martial law. While contributing thirty horse to the Scottish expedition in 1638, and lending the king £10,000 in 1639, he gave no support to the forced loan levied upon the city in the latter year.
Thomas Coventry, 5th baron (d. 1699), was created an earl in 1697 with a special limitation, on failure of his own male issue, to that of Walter, youngest brother of the lord keeper, from whom the present earl of Coventry is descended.