After graduation, as the political tension escalated between Great Britain and its North American colonies over issues such as trade, taxation, and governance, Winslow publicly put his support behind Massachusetts Governor Thomas Hutchinson and helped create "a company of Tories" in an effort to keep the peace.
By 1774, Winslow was forced to flee Plymouth, and in April 1775, as a member of the British Army, he was involved in a bloody confrontation at Lexington against the Patriot militia.
In March 1776, he retreated along with the British troops to Halifax, and by July had been appointed muster master general for the Loyalist forces.
After the Patriots' victory was secured in 1783, Winslow and his fellow Loyalists were essentially reduced to the status of refugees.
Frustrated with the efforts of the Nova Scotia authorities at settling the approximately 35,000 Loyalists, Winslow began to advocate for the creation of a separate colony in the area north of the Bay of Fundy.
Before his appointment to the New Brunswick Supreme Court in 1807, Winslow served in a number of government posts: surrogate general, judge of the inferior court of common pleas, muster master of the King's New Brunswick regiment, secretary to the International Boundary Commission, and deputy surveyor of the King's Woods.