Some sources list Sherman Minton, who took one of two seats vacated by deaths in 1949, as being Catholic. That would mean the longest gap without a Catholic on the court since the 19th century was the 12-week period between the November 1939 death of Pierce Butler, the fourth Catholic on the court, and the swearing-in of Frank Murphy in February 1940.
However, Minton didn't become a Catholic until 1961, after he retired from the court in 1956.
Minton's seat in turn went to Brennan, who would be the sole Catholic on the court for 30 years. He was joined by Antonin Scalia in 1986, and two years later by Anthony Kennedy.