In June 1644 Browne became a Major General for the parliamentary Oxfordshire, Buckinghamshire and Berkshire Regiments and set up his headquarters at Abingdon in order to harass the King's men at nearby Oxford. He was at the Siege of Oxford and received King Charles upon his hand-over by the Scots. He was elected Member of Parliament for Wycombe in October 1645. He became an alderman of the City of London for Langbourn ward on 29 June 1648 and was Sheriff of the City of London to 11 December 1649. He was secluded under Pride's Purge in December 1648, and was imprisoned for a period after being accused of conspiracy with the Scots.