Christiancy was born near Johnstown, New York in what is now Bleecker, New York to parents of humble means. His grandfather, Isaac Peckham, was one of the first pioneers in Caroga, New York, settling in the area as early as 1783. Christiancy attended the common schools and the Johnstown and Ovid Academies. After his father died when he was 13, he also had to support his family. He taught school and studied law. In 1836, Christiancy was admitted to the bar after moving to Monroe, Michigan, where he obtained a clerkship in a Federal land office. He married Elizabeth E. McClosky on November 16, 1839.

He was prosecuting attorney for Monroe County, Michigan from 1841 to 1846. In 1848 he was a delegate to the Buffalo, New York Free Soil Party convention, having left the Democratic Party over the question of slavery.

He was a member of the Michigan State Senate from 1850 to 1852 and an unsuccessful Free Soil Party candidate for Governor in 1852. He helped to organize the U.S. Republican Party in Jackson, Michigan in 1854. He purchased the Monroe Commercial in 1857 and became its editor. He was also an unsuccessful candidate for the U.S. Senate in that year.

Pursuant to the new state constitution adopted in 1850, the Michigan Legislature created a permanent State Supreme Court in 1857. Christiancy was elected as an associate judge of this first permanent Michigan Supreme Court. He was reelected twice and served until February 27, 1875, when he resigned to take the office of U.S. Senator. He served as Chief Justice from 1872 to 1874. Christiancy is known as one of the "Big Four" of Michigan judicial history for his service while on the court.

He was elected as a Republican to the U.S. Senate in 1874, defeating the incumbent Radical Republican Zachariah Chandler, and served in the 44th and 45th Congresses from March 4, 1875, to February 10, 1879, when he resigned due to ill health.

He served as Envoy Extraordinary and Minister Plenipotentiary to Peru from 1879 to 1881, after which he returned to Lansing, Michigan to resume the practice of law.

He died in Lansing and is interred in Woodlawn Cemetery in Monroe.

His son, James Isaac Christiancy (1844-December 18, 1899), was First Lieutenant, United States Army. He was awarded the Medal of Honor during the American Civil War while serving in Company D, 9th Michigan Volunteer Cavalry. On May 28, 1864, while acting as aide, he voluntarily led a part of the line into the fight, and was twice wounded. The Medal was actually issued on October 10, 1892. He is buried in Section 1 of Arlington National Cemetery.

This article incorporates facts obtained from the public domain Biographical Directory of the United States Congress.