McNamara held numerous Church and civic posts in the Kansas City area. He was a parish priest and high school teacher, Diocesan Refugee Resettlement Director, Chairman of the United Campaign Agency Executives Association, Chaplain of Jackson County Jail, President of the Kansas CityCitizens' Alliance for the War on Poverty. He was also a board member of the Human Resources Commission of Kansas City, State Committee on Aging, and the Jackson CountyCivil Rights Commission and was Moderator of the Diocesan Family Life Bureau. McNamara was known for his work in reorganizing and enlarging the scope of the Catholic Charities of the Diocese of Kansas City-St. Joseph. Under his direction of the agency, its efforts expanded to include programs in job opportunity training, remedial and adult basic education, tutoring for children in both Catholic and public schools, medically related services, family enrichment and pre-Cana programs, services to the elderly and housing programs. His agency sponsored the first Out of School Neighborhood Youth Corps Program in Kansas City. He also sponsored a community action program through the Office of Economic Opportunity to provide social work service to adolescent youth and teenage gangs, and a program for the training of unemployed adults. McNamara was chairman of the National Conference of Catholic Charities Commission on Housing from 1969 to 1972. He was the Diocesan Director for Catholic Relief Services, the overseas aid agency of American Catholics. In the latter capacity, he was sent in 1970 on a visitation of CRS programs in West Africa. He was appointed Executive Director of the Campaign for Human Development, United States Catholic Conference, in 1973 and served in that capacity for some five years.
On January 10, 1978, McNamara was appointed the sixth Bishop of Grand Island, Nebraska, by Pope Paul VI. He received his episcopal consecration on the following March 28 from Archbishop Daniel Sheehan, with Bishops Charles Helmsing and John Sullivan serving as co-consecrators. Subsequent to his ordination as bishop, he served as president of the National Catholic Rural Life Conference, President of the National Council of Catholic Bishops' Committees for Liaison with Women Religious, the American Board of Catholic Missions, Campaign for Human Development. In his later years, he served director of Catholic Relief Services; however, he stepped down in 1997 after African American staff members became outraged by his use of a racial slur. After reaching the mandatory retirement age of 75, McNamara resigned as Bishop of Grand Island on October 14, 2004. He died two months later, aged 76.
