Previously, McNamara served in the Louisiana House of Representatives from populous Jefferson Parish in suburban New Orleans. Elected in the jungle primary of 1975, he was a Democratic representative from 1976-1977, and a converted Republican member from 1977-1980. 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 A. Carson of New Orleans. In 1980, McNamara supported the Ronald W. Reagan and George Herbert Walker Bush national ticket. President Reagan thereafter nominated McNamara for the bench on May 5, 1982, for the seat vacated by Judge Edward James Boyle, Sr., and the U.S. Senate quickly confirmed him.
The New Orleans-born McNamara graduated with a bachelor of science degree from Louisiana State University in Baton Rouge in 1959. He received his Juris Doctor degree from Loyola University in New Orleans in 1968.
McNamara served in the U.S. Navy from 1959-1962. From 1966-1968, he was a bailiff and law clerk for Judge Herbert W. Christenberry of the same court on which McNamara serves. He maintained a private law practice from 1968-1982. 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.
McNamara was succeeded full-time on the court by Judge Jay C. Zainey (born 1951), a nominee of President George W. Bush, who was confirmed on October 10, 2001. Judge McNamara resides in Metairie with his wife Alma J. McNamara. The couple has two sons: attorneys John Price McNamara (born 1964) and Dwight L. McNamara (born 1967), both of Metairie. Judge McNamara is Roman Catholic.