Born in
Galena,
Illinois, Hallett attended the
Rock River Seminary in
Mount Morris, Illinois. Hallett
read law to enter the bar in 1858. He was in private practice in
Chicago, Illinois from 1859 to 1860. He was a gold miner in what was then the western end of the
Kansas Territory in 1860, returning to private practice in
Denver,
Kansas Territory (unorganized territory from January 29, 1861,
Colorado Territory from February 28, 1861) from 1861 to 1865. He was a member of the Colorado Territorial Council from 1863 to 1866. He was in private practice in Denver, Colorado Territory in 1866. He was the Chief Justice of the
Colorado Territorial Supreme Court from 1866 to 1877. ==Federal judicial service==