His acting career with the Abbey Theatre began in September 1910 with a role in R. J. Ryan's The Casting-out of Martin Whelan.

MacNamara is still best known for his first novel, The Valley of the Squinting Windows, set in a fictional village called Garradrimna, which caused a furore in his native Westmeath on its publication.

He continued to write for many years after this controversial first work, and located most of his later fiction in Garradrimna, in the Irish Midlands.

His work was part of the literature event in the art competition at the 1924 Summer Olympics.

He died at his home on Gilford Drive in Sandymount, County Dublin in February 1963.