[3] For many years, Winslow was a law partner with Joseph V. Quarles, who had been Mayor of Kenosha, Wisconsin, in 1876–1877, and would later become a U.S. senator and federal judge.

In May 1891, following the death of Justice David Taylor, Governor George Wilbur Peck appointed Judge Winslow to the Wisconsin Supreme Court.

He was elected to a full term on the court in 1895, defeating a challenge from 5th circuit Judge George Clementson, and was subsequently re-elected without opposition in 1905 and 1915.

In 1887, when Winslow was a circuit judge, Wisconsin law permitted women to vote but only in elections "pertaining to school matters."

Winslow agreed but the Wisconsin Supreme Court reversed his decision, holding women could not vote except on ballots limited to school offices.

In a 1909 address to the Milwaukee Loyal Legion, Winslow explained that judges "are sworn to protect and support both the federal and state constitutions as they are, not as [Progressives] would like to see them."

He also urged conservatives to consider that "the rights and privileges once deemed essential to the perfect liberty of the individual are often found to stand in the way of the public welfare, and to breed wrong and injustice to the community at large.

"[13] Winslow continued his campaign and gained a national audience through speeches and articles in popular magazines and law journals.

[15] Winslow and his fellow justice Roujet D. Marshall engaged in a continuing debate over the proper interpretation of the federal and state constitutions in light of changing social needs.

Marshall argued that "preservation of liberty is given precedent [even] over the establishment of government" and that courts must ensure that reform laws did not infringe it.

By the time Winslow died in 1920, his philosophy of flexible constitutionalism and adaptation to modern conditions was generally accepted in Wisconsin.