He was made a captain in the New York Militia on January 5, 1758, and repelled a second attack on German Flatts in April of that year.

[2][3][4] Herkimer joined the Scottish Rite Freemasonry, being initiated in the St. Patrick's Lodge in Johnstown, New York.

After the split in which Loyalist militiamen from the area withdrew to Canada, he was commissioned a brigadier-general in the county militia by the Provincial Congress on September 5, 1776.

In June 1776, he led 380 men of the Tryon County militia to meet with the Mohawk chief Joseph Brant at Unadilla, New York.

Herkimer asked the Mohawk and five other Iroquois nations to remain neutral, while Brant countered that the Indians owed their loyalty to George III.

In spite of his injuries, he sat propped up against a tree, lit his pipe, and directed his men in the battle, rallying them to avoid two panicked retreats.