Prior to the battle of Ridgeway, the Fenian insurgents, led by Brigadier General John O'Neill, a former Union cavalry commander who had specialized in anti-guerrilla warfare in Ohio, secured boats and transferred some 800 men across the Niagara River, landing above Fort Erie, before dawn on June 1, 1866. An additional 200–400 Fenians and supplies crossed later during the morning and early afternoon until the US Navy gunboat, the USS Michigan, began intercepting Fenian barges at 2:20 p.m. — 13 hours after the first Fenian advance party landed in Canada.

An advance party of 250 men of Lieutenant Colonel George Owen Starr's 17th Kentucky Fenian Regiment landed in Canada at about 1:30 AM and raised a large Fenian green flag with a gold Irish harp some two hours in advance of O'Neill's main force. Starr's advance party rushed to seize the town, cut telegraph wires and take control of the railway yards south of Fort Erie by dawn as the rest of O'Neill's force was disembarking. U.S. authorities also allowed unarmed men to board the ferry from Buffalo and small boats freely crossed the Niagara River until the afternoon. It is estimated that at least 1,000 and possibly as many as 1,350 Fenians in total crossed during the first thirteen hours of June 1, but it is impossible to determine a precise number.

O'Neill spent the first day trying to rally the local citizenry to the Fenian cause and to commandeer supplies for his mission, but his force was plagued by desertions almost from the outset. By nightfall, O'Neill estimated that he had perhaps 500 men remaining in his camp. Later during the night, O'Neill was reinforced by an additional column of 200 Fenians who had been deployed earlier on a bridgehead at Black Creek guarding against an approach from Chippawa in the north, bringing his total strength at Ridgeway to at least 650 men.