Pennington seems to have been frequently at odds with his fellow officers: his arrival in America was signalized by a duel with the Royal Navy captain who brought him over, and in 1793, he was publicly reprimanded by the Duke of York for court-martialing an adjutant over a trifle.
His last command was a Royal Veteran Battalion in 1806, which he resigned in 1813 upon inheriting his brother's barony and estates, dying five years later.
A contemporary letter by Horace Walpole, presumably reflecting the report of the time, attributes the quarrel to "humming a tune".
Primrose Kennedy all testified that Tollemache had verbally abused Pennington during the voyage, and expressed a desire to kill him in a duel.
Later accounts of the event became more fanciful, relating that Pennington and Tollemache exchanged fire with a brace of pistols before resorting to smallswords, and attributing the quarrel to a sonnet made by Pennington on Tollemache's wife; these embellishments do not appear to be supported by contemporary accounts.
At the Battle of Raismes on 8 May 1793, he led the battalion against La Marlière's division, driving them from the forest of Vicoigne and back to their entrenchments.
[11] Pennington displayed an irascible temper throughout the campaign, which steadily worsened and markedly impaired his relations with the other Guards officers.
During the Siege of Valenciennes, matters came to a head when he requested the court-martial of Captain Wynyard of the Coldstream Guards, an equerry to the Duke of York, over a triviality.
[citation needed] In 1806, he was appointed colonel of the 10th Royal Veteran Battalion, raised to provide garrison troops in Canada, and on 25 April 1808, he was promoted to general.
[19] In 1813, he succeeded his elder brother as Baron Muncaster by special remainder, and to the family estates in Cumberland and Yorkshire, subsequently resigning his colonelcy.