His parents sent him to boarding school in an effort to stymie his interest in acting, but McNamara won out. He spent a summer as an intern at the prestigious Williamstown Theater Festival before landing his first feature role in the youth-oriented drama "The Beat" (1987). He went on to appear as a younger version of Mark Harmon in "Stealing Home" (1988), Trini Alvarado's suitor in "Stella" and Jeff Bridges' chip-off-the-block profligate son in "Texasville" (both 1990) and a pivotal role in "Copycat" (1995). McNamara appears as a Hollywood star who, after joining the R.A.F. as a publicity stunt, is captured and held in an Irish prisoner of war camp in the WWII-era drama "The Brylcreem Boys", and as an adoptee who seeks revenge on his birth parents in "Natural Enemy" (both scheduled for release in 1996).
McNamara is perhaps best known for his diverse TV roles. In his TV debut, "Soldier Boys" (CBS, 1987), he was one of several teens who confuse the fantasy of war games with reality. Diane Keaton directed him in "Wildflower" (Lifetime, 1991) as a 1930s teenager who discovers an epileptic (Patricia Arquette) being held captive by her stepfather. McNamara gave two memorable performances in 1992: as the perfect son who couldn't face telling his parents he is gay in "Doing Time on Maple Drive" (Fox) and as Sharon Gless' murderous son in the true crime-inspired "Honor Thy Mother" (CBS). He also gained some notice as Montgomery Clift in the TV biopic, "Liz: The Elizabeth Taylor Story" (NBC, 1995).