Phelps completed his legal education in the offices of Samuel Ingham and Isaac Toucey, was admitted to the bar in 1845 and commenced practice in Essex, Connecticut.

Phelps served as judge of the superior court of Connecticut from 1863 to 1873.

He served as judge of Connecticut's supreme court of errors from 1873 until his resignation in 1875.

He declined to be a candidate for renomination in 1882, and resumed the practice of law.

He served as a delegate to several Democratic state conventions.