Major Mercer (Worcestershire horse)
Major Mercer, the commander of the Worcestershire horse, played a significant part in the Battle of Worcester in 1651, but although mentioned in primary sources his full name is not given in them. David Laing speculated that he was John Mercer, the younger brother of William Mercer who was also a parliamentary officer and author of "Angliae Speculum, or England's looking-glass" (London 1646).
Major Mercer, the commander of the Worcestershire horse, played a significant part in the Battle of Worcester in 1651, but although mentioned in primary sources his full name is not given in them. David Laing speculated that he was John Mercer, the younger brother of William Mercer who was also a parliamentary officer and author of "Angliae Speculum, or England's looking-glass" (London 1646).

Notes
References
Atkinson, Charles Francis (1911), "Great Rebellion#59 The 'Crowning Mercy'" , in Chisholm, Hugh (ed.), Encyclopædia Britannica (11th ed.), Cambridge University Press, pp. 420–421
Baldock, Thomas Stanford (1809), Cromwell as a Soldier, K. Paul, Trench, Trübner & Company, pp. 505, 508
Laing, David (1862), "Some account of Lieut.-Colonel William Mercer, Author of  'Angliæ Speculum, OR ENGLAND'S LOOKING-GLASSE', London 1646" (PDF), Proceedings of the Society of Antiquaries of Scotland, Society of Antiquaries of Scotland, 3: 348–357
Stace, Machell (1810), Cromwelliana. A chronological detail of events in which Oliver Cromwell was engaged; from the year 1642 to his death 1658: with a continuation of other transactions, to the restoration, Westminster, pp. 111–112