McCarthy was appointed to the Supreme Court bench in 1957, and from 1958 onwards he served periodically as a temporary judge of the Court of Appeal of New Zealand. Sir Thaddeus was President of the Court of Appeal from 1973 to 1976, and was the presiding member of an unprecedented seven Royal Commissions. He was also a Commissioner of Security Appeals.
McCarthy had a reputation as a "plain English" judge with an outstanding ability to make complex matters understandable. He was awarded the Order of New Zealand on 6 February 1994.