US Congressman. He was born one of two children (he also had a sister Alice Margaret born in 1885) in Derby, Connecticut, to Thomas Jefferson O'Sullivan and Margaret A. Brett O'Sullivan, and attended and was educated in local public schools before entering the distinguished Yale University in New Haven, Connecticut, where he graduated from in 1908. He also attended Georgetown University in Washington, D.C, and graduated from there in 1909, and studied law at the Yale Law School in New Haven, Connecticut, where he graduated from in 1913. On June 18, 1913, he married Margaurite G. Lawton in Middletown, Connecticut, and they had at least four children including two sons, Thomas Jefferson O'Sullivan was born in 1914 and James Lawton O' Sullivan was born in 1916. Following his education, he was admitted to the bar in 1923 and commenced to practicing in his birthplace of Derby, Connecticut. He then served as Corporation Counsel of Derby, Connecticut, from 1914 to 1917. He served as a Delegate to the Democratic National Convention in 1916, and as a Member of the Connecticut State Senate and its Minority Leader in 1917. He resigned from that post to serve in the war in 1918. During World War I, he served in the United States Navy. After his military service, he decided to run for a seat in the United States Congress and was elected. A Democrat, he then served Connecticut's 5th District (Sixty-Eighth Congress) in the United States House of Representatives from 1923 to 1925. He was an unsuccessful candidate for reelection to the Sixty-Ninth Congress in 1924. After his term in the United States Congress expired on March 3, 1925, he was succeeded in office by United States Representative James Peter Glynn. He then continued with his practice of law and served as an associate professor at the Yale Law School in New Haven, Connecticut. He served as a Connecticut State Chair in 1930. He then served as a Judge of the Connecticut Superior Court from 1931 to 1950, Associate Justice of the Connecticut Supreme Court from 1950 to 1957, and as Chief Justice from April 9, 1957, to August 11, 1957. He had to retire from the post of Chief Justice on August 11, 1957, due to that he had reached the mandatory age for retirement. He did, however, continue serving as a Connecticut State trial referee. He lastly served as a Delegate to the Connecticut State Constitutional Convention representing its 3rd District in 1965. He retired from public life shortly thereafter and lived in Orange, Connecticut, until his death at the age of 91. His funeral was held at Sisk Brothers Funeral Home in New Haven, Connecticut, and he was buried next to his wife Margaurite who died in 1967 in the St. Lawrence Cemetery in West Haven, Connecticut.