Thelma's Schoonmaker's father Bertram was employed as a clerical worker by the Standard Oil Company and worked abroad.[4] She was born in Algiers, Algeria to American expatriates and raised in various countries, including on the Dutch-Caribbean island of Aruba.[4][5]

Schoonmaker did not live in the United States until she was a teenager in 1955, and was initially alienated and dumbfounded by American culture.[4] Schoonmaker was interested in a career in international diplomacy and began attending Cornell University in 1957, where she studied political science and the Russian language. (She attended classes taught by Vladimir Nabokov.) When she graduated from Cornell in 1961, she began taking State Department tests in order to apply for positions in the U.S. government.[3][4] Being politically inclined and opinionated, Schoonmaker expressed distaste for the South African policy of apartheid, a stance which didn't sit well with those administering the State Department tests.[1][4] In reaction to this experience, Schoonmaker switched gears and began taking a course in primitive art.

At that point, she saw an employment advertisement in The New York Times which sought an "assistant film editor" to which she responded and got the job. The job entailed assisting an "editor" who was randomly cutting frames from classic European films, (such as those by François Truffaut, Jean Luc Godard and Federico Fellini), so that their length would conform to the running times of U.S. television broadcasts.[4] Although she was revolted by the callousness of the editor's methods, Schoonmaker nonetheless picked up important technical skills, including negative cutting.

Schoonmaker signed up for a brief six-week course in filmmaking at New York University (NYU), where she came into contact with budding young filmmaker Martin Scorsese, who was struggling to complete his film What's a Nice Girl Like You Doing in a Place Like This? A negative cutter had butchered his film, not leaving enough negative frames to allow for hot splicing. One of her film professors asked Schoonmaker to help Scorsese; a close working relationship with him has unfolded over the past thirty-five years.[4]

At NYU Schoonmaker also met director Michael Wadleigh and later edited his influential music festival documentary, Woodstock. Her first major film editing work on Woodstock garnered Schoonmaker an Academy Award nomination for Best Editing. Her careful use of superimpositions and freeze frames brought the performances in the film to life, and added to the movie's wide appeal, thus helping to raise the artistry and visibility of documentary filmmaking to a new level.[6]

Despite her obvious skill and talent, the early period of Schoonmaker's career was difficult; joining the Motion Picture Editors Guild has always been challenging, and the film industry generally has been a restrictive boys club.[2][5] Consequently, there was a twelve-year gap between her work on Scorsese's student films and her Oscar-winning work on his masterful Raging Bull.

Nonetheless Schoonmaker helped to shatter some film industry glass ceilings, and has amassed an impressive list of film editing credits. Variety's Eileen Kowalski notes that, "Indeed, many of the editorial greats have been women: Dede Allen, Verna Fields, Thelma Schoonmaker, Anne V. Coates and Dorothy Spencer."[7]