Collins practiced law in Cleveland for several years, then moved to the Wisconsin Territory in 1842, settling in Madison. 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, through which Collins became a prominent and well-respected lawyer in the state. Some of his siblings also relocated to Wisconsin—his older sister, Sarah, was the wife of Wisconsin Territorial Governor James Duane Doty. 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. He served on the Council until its dissolution, at the time Wisconsin became a state. In Wisconsin's first election for voting members of Congress, Collins was the Whig Party's candidate in the 2nd Congressional district. He was defeated in that election by Democrat Mason C. Darling. After the seating of the 1st Wisconsin Legislature, Collins stood, along with Edward V. Whiton, as Whig candidates for United States Senate—U.S. senators at the time were elected by the state legislature, rather than through popular election. The Democratic Party had a clear majority in the Legislature, thus Collins and Whiton were defeated by the Democratic Party ticket of Henry Dodge and Isaac P. Walker. Later that year, Collins was appointed to the first Board of Regents of the University of Wisconsin. Whiton went on to service on the Wisconsin Circuit Court, and later became the first Chief Justice of the Wisconsin Supreme Court. The following year, in 1849, Collins was the Whig Party's nominee in the 2nd Wisconsin gubernatorial election, running against incumbent Governor Nelson Dewey. Collins was defeated and Dewey went onto another term as Governor. Two years later, in 1851, Collins was, again, the Whig nominee for United States Senator, running against incumbent Democrat Henry Dodge. The Democratic Party still held a significant majority in the 1851 legislature, thus re-elected Dodge. In 1852, Collins became Chairman of the Whig Party in Wisconsin, and served as a delegate to the 1852 Whig National Convention, in Baltimore. The 1852 convention would turn out to be the last Whig National Convention. At the convention, Collins was a strong supporter of Daniel Webster, who had been supportive of the Compromise of 1850 and associated with the enforcement of the Fugitive Slave Act of 1850. When the convention chose General Winfield Scott as its nominee, Collins left in disgust. As the Whig Party dissolved into the newly forming Republican Party, Collins became briefly associated with the Democratic Party. In 1855, with support of the Democratic Party, he was elected Circuit Court Judge for the newly created 9th Circuit. He had to resign a few years later, in the Fall of 1858, due to health problems.
Later years
After leaving the court, he went into the land business with his brother-in-law, former Territorial Governor James D. Doty, in Menasha, Wisconsin. At the start of the American Civil War, their business faltered, and in 1864, Collins traveled to California for health and recreation. He returned to Wisconsin in 1867, and, in 1874, resumed his law practice in Appleton, Wisconsin.
Death
In his later years, Judge Collins resided with his son, Alexander, Jr., in Neenah, Wisconsin, and his daughter, Mrs. Jessie McCord, in Milwaukee. He died in Neenah on October 31, 1901.
