He attended Union Academy at Belleville, and graduated from Union College in 1833.
Then he taught school and was Principal of Union Academy, and subsequently taught at Watertown Academy. Then he studied law, and was admitted to the bar in 1837. Mullin was District Attorney of Jefferson County from 1843 to 1845.
He was elected as a Whig to the 30th United States Congress, holding office from March 4, 1847, to March 3, 1849.
He was President of the Village of Watertown in 1853 and 1854. In 1855, he ran on the Republican ticket for the New York Court of Appeals, but was defeated by the American Party candidate George F. Comstock.
He was a justice of the New York Supreme Court (5th District) from 1858 to 1881, and was ex officio a judge of the Court of Appeals in 1864. He was buried at the Brookside Cemetery, in Watertown.
Member United States House