The Canada Militia Act of 1855 reorganized the whole sedentary militia into an active militia of 5,000 men. Following the Trent Affair in 1861, tensions peaked and the threat of Fenian raids in Canada was real. With the American Civil War in full swing, these Irish sympathizers planned to invade Canada to divert Britain's attention away from problems at home. All towns along the US-Canada border organized and trained their local voluntary militia units. With supplies, weapons, and uniforms shipped from England, ordinary citizens became soldiers ready to defend their towns and villages.

On December 11,1862, the Brockville Infantry Company was organized under the command of Captain Jacob Dockstader Buell, a lawyer. The company comprised 3 officers and 55 militiamen. In 1863 five other neighbouring infantry companies from Lyn, Farmersville, Mallorytown, Easton's Corners and Burritt's Rapids joined to form the 23rd Battalion of volunteer militia, with headquarters in Brockville. The Brockville Infantry Company was called into active service in 1866 during the Fenian raid following the Battle of Ridgeway, but no shots were ever fired. Reorganization on October 5,1866 changed the name to the 42nd Battalion.
With Confederation being proclaimed on July 1,1867, the defence of the country no longer relied on the infantry companies, but became the responsibility of the regular army. Later the headquarters moved to Perth, then to Renfrew, then to Pembroke and the name became The Lanark & Renfrew Scottish Regiment. Today the regular and volunteer militia is known as The Brockville Rifles.