He appeared in a number of leading Australian cases, including the constitutional case of Mooraberrie Cattle case At the swearing in of Thomas William McCawley as a judge of the Supreme Court, Macrossan was one of the few judges to argue that McCawley's commission as a judge of the Supreme Court of Queensland was valid. That issue went to the High Court of Australia and later to the Privy Council which determined that the appointment of McCawley was valid. It was during his time at the bar that his first wife Lydia died in 1922. Macrossan was sworn in as a judge of the Supreme Court of Queensland on 23 July 1926. He became senior puisne judge on 1 December 1926. The following day, he married Gladys Mildred Trenfield at Fortitude Valley. They separated 10 years later. In 1929, Pope Pius XI appointed Macrossan a Knight of St Gregory the Great. Macrossan had a "tongue of silver and steel" and was noted for his hard work and speedy judgments. Unfortunately, his wit was not always appreciated. In hearing a divorce suit between two unmarried people who were living together as a married couple, Macrossan remarked "I suppose they will go and get married and spoil it all". Unfortunately for Macrossan, two ministers heard his remark and wrote a letter of complaint to the local paper protesting at the judge's comments about the institution of marriage. Macrossan took the letter as being a contempt of court by the ministers as well as the newspaper, and he duly convicted each of them. An appeal to the Full Court of the Supreme Court overturned the convictions for contempt. Macrossan was appointed as Chief Justice of Queensland on 17 May 1940. He was unable to make any great changes as he died in office shortly thereafter. He was buried in Nudgee Cemetery. His younger brother Neal William Macrossan became chief justice in 1946 and his nephew John Murtagh Macrossan (son of Macrossan's brother Vincent) also became chief justice in 1989.