He was Chief Judge of the New York Court of Appeals from 1945 until his death. He was a justice of the New York Supreme Court from 1931 to 1934. On May 21, 1934, he was appointed to the New York Court of Appeals to fill the vacancy caused by the resignation of Henry T. Kellogg.
In November 1934, he was elected on the Democratic and Republican tickets to a full 14-year term.
In September 1945, after the death of Irving Lehman, Loughran was appointed Chief Judge of the Court of Appeals. In November 1946, he was re-elected unopposed to a 14-year term as Chief Judge, and died in office.
He died of a heart attack, and was buried at the Saint Mary"s Cemetery in Kingston, New York
The John T. Loughran Bridge was named in his honor.