George Lucas's first feature film after his student short was THX 1138. The 1971 science-fiction film expanded Lucas's 1967 USC student short, Electronic Labyrinth: THX 1138 4EB, into a feature-length story about a highly controlled dystopian society.
Lucas directed the film and co-wrote it with Walter Murch. Francis Ford Coppola produced it through American Zoetrope, the filmmaking company they had formed with other collaborators. Robert Duvall starred as THX 1138, while Donald Pleasence played SEN and Maggie McOmie played LUH.
THX 1138 was an important artistic beginning, but it was not Lucas's commercial breakthrough. The film received critical attention yet performed poorly financially. American Graffiti, released in 1973, reached a much larger audience and established Lucas as a major filmmaker. Willow came much later, and Star Wars followed in 1977, so neither can be the answer to this question.