Hun gik Tekstil High School i New York.<ref name=":2">Brettell, Andrew; King, Noel; Kennedy, Damien; Imwold, Denise; Leonard, Warren Hsu; von Rohr, Heather (2005). ''Cut!: Hollywood Murders, Accidents, and Other Tragedies''. Barrons Educational Series. s. 277. {{ISBN|0-7641-5858-9}}.</ref> Som teenager blev McNamara opdaget, da modelagent [[John Robert Powers]] så billeder af hende taget i en vens hjem. Med sin mors støtte underskrev McNamara med sit agentur og begyndte at arbejde som teenage-model, mens hun stadig gik i [[Gymnasium|gymnasiet]]. Hun var en af de mest succesrige teenage-modeller i tiden og optrådte i [[Seventeen (magasin)|Seventeen]], [[LIFE]], [[Harper's Bazaar]] og [[Vogue]].<ref name=":3">"Crazy Things Do Happen To Maggie McNamara". The Palm Beach Post. 28. januar 1962 s. 25</ref> McNamara kommenterede senere på sine dage som model:
{{Citat|I was terribly shy, and I used to work on myself to keep from showing it. When I was facing a camera, I pretended that neither it nor the photographer were there. I played a game with myself according to the clothes I was wearing...You have to feel right in what you are wearing, to have it look right. Just as each period has its own fashion, each person has his own style. When you find it, I think you should stay with it. When I was modeling I had to dress exactly as Vogue wanted the picture to be. But any good quality becomes something else when it is overdone, and I feel that this applies to being too clothes conscious. I don't care what the fashion dictator says. I will not follow if it's not right for me. But your overall impression consists of more than clothes. Your grooming, posture, the sound of your voice, and your perfume play a part in the total picture you create.<ref name=autogeneret1 />}}
I april 1950 optrådte McNamara på coveret af [[LIFE]] for anden gang. Efter at have set hende på coveret, tilbød producer [[David O. Selznick]] hende en film kontrakt. Hun afslog tilbuddet og fortsatte som model, mens hun studerede dans og skuespil.<ref name=":1" /><ref name=":3" />
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