He was the son of Supreme Court Justice Joseph Mullin (1811–1882).
He attended the public schools, and graduated from Rensselaer Polytechnic Institute in 1869.
He was admitted to the bar, and practiced law in Watertown.
He was a member of the New York State Senate from 1892 until his death in 1897, sitting in the 115th, 116th (both 21st D.), 117th, 118th (both 22nd D.), 119th and 120th (both 35th D.).
He died on September 1, 1897, in his bedroom at the University Club in New York City, of "heart disease", and was buried at the Brookside Cemetery in Watertown.