Public office
Browne probably served as a Justice of the Peace in Essex, his wife's county, however, and had been named in 1630 for the sewers commission in Bedfordshire.
He was returned as a member for the boroughs of Clifton-Dartmouth-Hardness, in the Long Parliament of November 1640. He appears to have had no connection with the constituency before he was elected and he may have gained it through the patronage of his cousin Oliver St John who was MP for Totness and had been in Lincoln's Inn at the same time as Browne.
In February 1643—possibly through the influence of his cousin St. John (who was then solicitor-general)—Browne, Serjeant Richard Creswell and John Puleston, were recommended by the parliament to be Barons of the Exchequers, in the peace proposals laid before the king at Oxford, which came to nothing. Around the same time he joined the newly formed Committee of Both Kingdoms on which he continued to sit until 1648, and the Committee for the Preservation of the Records. In November of that year Browne and St. John were two of the four members of the House of Commons to whom, with two lords, the new Great Seal was entrusted.
The commoners appointed as commissioner of the Great Seal still continued to perform their other parliamentary functions. Lord Commissioner Browne was most active in the proceedings against Archbishop Laud, summing up the case in the House of Lords and carrying up the ordinance for his attainder passed by the Commons in November 1644. His speech has not been preserved, but from the constant references which Laud makes to it he appears to have put the case against the archbishop in a very effective way.
After the trial was ended (2 January 1645) he was deputed, with Serjeants John Wilde and Robert Nicolas, to lay before the House of Lords the reasons which, in the opinion of the House of Commons, justified an ordinance of attainder against the archbishop. This had already been passed by the Commons, and the Lords immediately followed suit.
In July 1645 a paper was introduced to the House of Commons, emanating from Lord Savile, and containing what was in substance an impeachment of Denzil Holles and Bulstrode Whitelocke, of high treason in betraying the trust reposed in them in connection with the recent negotiations at Oxford, of which they had had the conduct. After some discussion the matter was referred to a committee, of which Browne was nominated chairman. The affair is frankly described by Whitelocke as a machination of the independents, designed to discredit the Presbyterian party, of which both Hollis and himself were members; and as he accuses Browne of displaying a strong bias in favour of the impeachment, it may be inferred that at this time he had the reputation of belonging to the advanced faction. The charge was ultimately dismissed.
In 1646 Browne sat on the Committee for Exclusion from Sacrament and the Committee for the Abuse of Hereditary, and after remaining a commissioner of the Great Seal for nearly three years, the lords commissioners were removed in October 1646, and the Great Seal transferred to the speakers of the two houses. With his workload in Parliament reduced he resumed his practice at the bar.
In 1648 Browne sat on the Committee for Scandalous Offences and his time of sitting on the Committee of Both Kingdoms came to an end. He was also sent as one of the commissioners to treat with the king in the Isle of Wight. On the receipt of letters from the commissioners containing the king's ultimatum (the Treaty of Newport), the House of Commons, after voting the king's terms unsatisfactory, resolved "that notice be taken of the extraordinary wise management of this treaty by the commissioners".
The next day, 12 October 1648, he was included in the batch of twenty-two who were made Serjeants by the parliament, when both he and his cousin St. John were also elevated to the bench, he as judge of the King's Bench, and St. John as chief justice of the Common Pleas. With the failure of the Newport negotiations (and the reconstitution of the House of Commons by Pride's Purge), the House of Commons resolved to try King Charles for treason, Browne with five of his colleagues, resigned their seats on the bench rather than participate in the Regicide.
Browne took no further part in public life until the last year of the Interregnum. After the fall of the Protectorate Browne was elected to Parliament for the constituency of Bedford in 1659 and to Bedfordshire in the Convention Parliament of 1660.
After the restoration of the monarchy, he was not only immediately reinstated as a serjeant, and within six months was reinstated to a place on the bench, being constituted, on 3 November 1600, a judge of the Common Pleas. On 4 December of that year he was knighted. He retained his seat as a judge of the Common Pleas until his death on 11 April 1668. He was buried under a monument still existing in the church of Arlesey in Bedfordshire, where he had a house.