The royalist army which marched on London following the battle of Edgehill probably totalled around 12,000 men, but not all were engaged at Brentford. Working from a number of contemporary accounts and restoration petitions, elements from at least eight royalist foot regiments (Sir Thomas Salisbury, Earl Rivers, Sir Edward Fitton, Lord Molyneux, Sir Gilbert Gerrard, Thomas Blagge, John Belasyse and Sir Thomas Lunsford) and three cavalry regiments (Lord Grandison, Sir Thomas Aston and the Prince of Wales) can be identified as being directly involved in the battle.  This suggests that up to 3,800 royalist foot and 800 cavalry might have been engaged at Brentford; only perhaps one third of the forces available to the King.  The royalists also appear to have readied four cannon, two 4 pounder minions and two 9 pounder demi-culverins, each with 20 rounds of shot, for the advance on Brentford with the remainder of the artillery train following behind.

For parliament, New Brentford/Brentford End (the western part of modern day Brentford either side of the River Brent) was defended by Denzil Hollis’ regiment of foot consisting probably of around 800 men with Lord Brooke’s regiment of foot of 480 men seemingly in Old Brentford (the eastern part of modern day Brentford).  These regiments had arrived in Brentford by the morning of 11 November 1642.  Although the foot were supported by 10-12 troops of cavalry, most of the parliamentary horse fled on hearing of the royalist attack, leaving, apparently, only a troop of probably around 60 men commanded by Captain Robert Vivers.  The parliamentary forces also seem to have had two or three artillery pieces.   

The parliamentary forces suffered in the action. According to royalist accounts between six and eleven colours were captured, suggesting at least that many broken companies, and a number of officers, including a lieutenant colonel, some captains and other junior officers were killed. The burial registers of St Lawrence’s Church Brentford for November and December 1642 record the interment of two parliamentary captains and three lieutenants. The most senior parliamentary officer at Brentford, Lieutenant Colonel James Quarles of Lord Brooke’s regiment of foot, is also known to have been killed. At least two parliamentary officers were also captured, captains Robert Vivers and John Lilburne. Accounts of casualty and prisoner numbers vary, but it appears that around 50 parliamentary soldiers were killed in the battle, with perhaps more than this drowning during the rout. A contemporary, third hand, account suggests 140 parliamentary soldiers were killed in total, which appears credible. Others would have succumbed to wounds following the battle. Four hundred parliamentary prisoners were said to have been taken, but 140 were released shortly after the action; the remainder apparently enlisting with the royalists.   Perhaps the fact which best illustrates the impact of the battle is that by early 1643 Hollis’ regiment had been disbanded, seemingly as a result of the casualties it sustained at Brentford. 

Royalist losses are poorly recorded in contemporary accounts. One details the deaths of sixteen men, including one captain and two lieutenants. The King, in a subsequent letter to parliament, claimed ten royalist dead. One unsubstantiated account suggests the royalist dead were removed to Hounslow Heath.