Years before, he had opened the first FBI office in Maine.
During World War II, the FBI assigned him as the first agent in Suffolk County, where he patrolled the Long Island coast and apprehended a group of German spies targeting several New York City landmarks.
McNamara, who spearheaded the investigation, and other agents arrested several men, who were prosecuted and executed.
McNamara, the country's second oldest former FBI agent, died of natural causes Dec. 8 surrounded by his family at a hospice center in Vero Beach, Fla. He was 103.
Those who knew him best described McNamara as a devout Catholic. He leaned heavily on his faith when he lost his wife, Mary, and three of their seven children in a fire in their Bay Shore home in 1960 while he was at a religious retreat, family members said. The couple had been married 21 years.
"He was a man who had a deep Catholic faith, which was the bedrock of his life. He went to Mass every day, and it really sustained him in a lot of ways," said granddaughter Marybeth Christie-Redmond, 49, of Essex, Vt.
He later married Ursula Daly Amend of the Bronx, joining together their two families, he with four children, she with three. They were married for 39 years. She died in 2000.
McNamara graduated from Cathedral College in Flushing, Queens, in 1928. After a stint in seminary school, he pursued a law degree from St. John's University in Brooklyn.
After graduating in 1936, he joined the FBI and served 14 years as a special agent in Minneapolis, Atlanta and Boston. He then was assigned to open an office in Portland, Maine, becoming the state's lone agent.
He worked with Hoover twice in his career, the other involving two high-profile kidnapping cases.
After retiring from the FBI, McNamara began a law practice in Bay Shore, which he had for 50 years.
He was appointed special assistant to the U.S. Attorney General in 1952-53, prosecuting waterfront crime.
"The thing I'll miss is his robust zest for life. He was a wonderful conversationalist. Everyone he met, he was genuinely interested in. Because of his inquiring nature, it gave him a view on life most people don't see," said his son, J. Paul McNamara, 64, of Bethesda, Md.
Besides his son J. Paul, he is survived by his children MaryAnn Christie of Vero Beach, Fla.; Patricia Moser of Princeton, N.J.; Ursula Morris of Guilford, Conn.; Daniel Amend of Tarpon Springs, Fla., and Cathleen Callahan of Cold Spring Harbor.
A funeral is tomorrow at noon at St. Patrick Church, 9 N. Clinton Ave., Bay Shore.