She was born on June 18, 1928, in New York City, US, in the family of Irish American parents Timothy McNamara and Helen Fleming McNamara as one of their four children. When she was nine years old her parents divorced.

She studied at New York’s ‘Textile High School’. She began her professional career as a model in her teens while continuing with her studies in dance and drama.

Gradually she achieved fame as one of the most successful models in the agency of John Robert Powers. During such time, in April 1950, she featured on the cover of ‘Life’ magazine. Her photo was spotted by American film producer, film studio executive and screenwriter David O. Selznick who subsequently offered her a film contract, which she refused and went on with her modelling career while completing her dance and drama lessons.

She was spotted by director Otto Preminger when she was 23 and was soon selected to play the part of Patty O'Neill in the 1951 play ‘The Moon Is Blue’. She also played the part briefly in the play’s New York production. That year she also performed in the ‘Broadway’ play ‘The King of Friday's Men’ and received positive reviews.

When the play was adapted into a film by Preminger in 1953, she reprised her role of Patty O'Neill. The subject matter of the film that included erotism and bold use of words like “seduce”, “mistress”, “virgin” and “pregnant” drew tremendous controversy and was not given the seal of approval by the ‘Motion Picture Association of America’ (‘MPAA’). Although the American film and television entertainment studio ‘United Artists’ that produced the film went ahead with its release on July 8, that year, it soon faced ban in Maryland, Ohio and Kansas. ‘National Legion of Decency’ also gave the film a “Condemned” rating.

However amidst many complications and obstacles ‘The Moon Is Blue’ turned out to be a smashing hit at the box-office garnering $3.5 million and also brought instant fame to debutant McNamara, who earned a ‘BAFTA’ nomination as Most Promising Newcomer and also an ‘Oscar’ nomination as Best Actress.

Signed by ‘20th Century Fox’, she sealed her success reinforcing her popularity with yet another hit, the American romantic comedy, ‘Three Coins in the Fountain’ directed by Jean Negulesco that was released on May 20, 1954. It won two ‘Academy Awards’ for Best Cinematography and Best Song out of three nominations and also garnered positive response from the critics circle.

Her next film was a biographical, ‘Prince of Players’ regarding one of the most celebrated American actors of the 19th century, Edwin Booth. It was a ‘20th Century Fox’ film directed and produced by Philip Dunne and released on January 11, 1955. The film became a moderate success.

After ‘Prince of Players’, she somewhat faded away from Hollywood only to be seen later in what proved to be her final film, the 1963 drama ‘The Cardinal’ directed by Otto Preminger. She was seen essaying a supporting role as Florrie Fermoyle, one of the sisters of the protagonist in the film.

While some cite her disturbed conjugal life and subsequent divorce with actor/director David Swift as the reason for her decline in Hollywood, others attribute her refusal for her films publicity and other such issues to be the reason for her downfall. Director Otto Preminger’s 1977 memoir emphasize that "Maggie suffered greatly after becoming a star. Something went wrong with her marriage to director David Swift. She suffered a nervous breakdown”.

She was however seen in the 1962 ‘Broadway’ play ‘Step on a Crack’. Since 1963 she started focussing on television work and was soon seen in the American medical drama series ‘Ben Casey’ as Dede Blake in its episode ‘The Last Splintered Spoke of the Old Burlesque Wheel’. She also played Barbara "Bunny" Blake, the title character of the episode ‘Ring-a-Ding’ in the American television anthology series ‘Twilight Zone’ that year.

She last appeared on television in 1964 doing single episodes of three series namely ‘The Great Adventure’ as Laura Drake in its ‘The Colonel from Connecticut’ episode; ‘The Greatest Show on Earth’ as Moira O'Kelley in its ‘Clancy’ episode; and finally in July that year in ‘The Alfred Hitchcock Hour’ as Pamela in its ‘Body in the Barn’ episode.

Little was known of her following her retirement from acting in the early 1960s apart from that she was working as a typist in New York City. It came to light only after her death through one of her relatives that she was penning down a film script titled ‘The Mighty Dandelion’, which probably was accepted by a new film company.

She married actor/director David Swift in March 1951 but the marriage culminated into divorce. The couple together had no children.

Although McNamara never married again, she had a romantic relationship with screenwriter Walter Bernstein.

Her dead body was found lying on her apartment couch in New York City on February 18, 1978, with a note of suicide on her piano. She took an overdose of sleeping pills to end her life a few months before turning 50.

Her remains were buried in ‘Saint Charles Cemetery’ located in Farmingdale, New York.