McNamara, a son of Henry D. and Ruby (née Price) McNamara, graduated in 1954 from the Roman Catholic Jesuit High School in his native New Orleans, Louisiana. In 1959, he received a Bachelor of Science degree in engineering from Louisiana State University in Baton Rouge. In 1968 he received his Juris Doctor from Loyola University New Orleans College of Law.
McNamara served in the United States Navy from 1959 to 1962. From 1966 to 1968, while in law school he was a bailiff and law clerk for United States District Judge Herbert W. Christenberry of the same court on which McNamara served. He maintained a private law practice from 1968 to 1982.
In 1976, he joined three other attorneys in forming the partnership Hailey, McNamara, McNamara, and Hall. The name was changed to Hailey, McNamara, Hall, Larmann & Papale in 1982, when A. J. McNamara was appointed to the district judgeship. The firm is the largest in Jefferson Parish and one of the largest litigation operations in metropolitan New Orleans.
While practicing law, McNamara served in the Louisiana House of Representatives from populous Jefferson Parish in suburban New Orleans. Elected in the first-ever nonpartisan blanket primary, also called the jungle primary of 1975, he was a Democratic representative from 1976 to 1977, and a Republican convert from 1977 to 1980. He did not seek reelection in the 1979 primary.
In 1976, as a Democrat, McNamara cast a critical vote against the Equal Rights Amendment in the House Civil Law Committee. The unfavorable committee vote effectively killed the prospects of ratifying the amendment for the second time in Louisiana.
In the 105-member Louisiana House, McNamara served with six other Republicans at the time: Clark Gaudin of Baton Rouge, three Caddo Parish members, Art Sour, B.F. O'Neal, Jr., and Bruce Lynn, and two converts to GOP, Michael F. "Mike" Thompson of Lafayette, and Lane Carson of New Orleans.
In 1980, McNamara supported the Ronald Reagan and George H. W. Bush national ticket. McNamara was nominated by President Reagan on May 5, 1982, to a seat on the United States District Court for the Eastern District of Louisiana vacated by Judge Edward James Boyle Sr. He was confirmed by the United States Senate on June 18, 1982, and received commission on June 21, 1982. He served as Chief Judge from 1999 to 2001. He assumed senior status on June 9, 2001. His service terminated on December 2, 2014, due to death. McNamara was succeeded full-time on the court by Judge Jay C. Zainey, a nominee of President George W. Bush, who was confirmed on October 10, 2001.
McNamara resided in Metairie, Louisiana with his wife, the former Alma J. Loisel. The couple had two sons, John Price McNamara and Dwight Loisel McNamara (1967-2005), and two daughters, Joni McNamara Parent and Nancy Ann McNamara Miller (died 2014).
McNamara died at his home of progressive supranuclear palsy in 2014. He was interred on December 6 at Greenwood Cemetery, New Orleans.