He continued his law practice there, first establishing a brief partnership with Thomas W. Sutherland, who had been the United States Attorney for the Wisconsin Territory.

He soon moved into a longer-term legal partnership with George Baldwin Smith, and, after 1853, with Elisha W. Keyes,[1] through which Collins became a prominent and well-respected lawyer in the state.

[2] Some of his siblings also relocated to Wisconsin—his older sister, Sarah, was the wife of Wisconsin Territorial Governor James Duane Doty.

[3] In 1846, Collins was elected to his first public office when he was chosen to represent Dane, Green, and Sauk counties on the Territorial Council.

In Wisconsin's first election for voting members of Congress, Collins was the Whig Party's candidate in the 2nd Congressional district.

The following year, in 1849, Collins was the Whig Party's nominee in the 2nd Wisconsin gubernatorial election, running against incumbent governor Nelson Dewey.

[2] Two years later, in 1851, Collins was, again, the Whig nominee for United States Senator, running against incumbent Democrat Henry Dodge.

At the start of the American Civil War, their business faltered, and in 1864, Collins traveled to California for health and recreation.

In his later years, Judge Collins resided with his son, Alexander, Jr., in Neenah, Wisconsin, and his daughter, Mrs. Jessie McCord, in Milwaukee.