After his education and spending 13 months in Europe "absorbing the revolutionary changes occurring in traditional stage design", in 1923 he worked for the Theatre Guild in New York as an assistant stage manager and bit actor. Mielziner's Broadway debut as a designer was in 1927 with "The Guardsman", for which he designed the scenery and lighting. His other Broadway credits include the original productions of "Sweet and Low", "Of Thee I Sing", "Another Part of the Forest", "Winterset", "Oh, Captain!", "Dodsworth", "Strange Interlude", "Carousel", "South Pacific", "Guys and Dolls", "The King and I", "A Streetcar Named Desire", "Death of a Salesman", "Cat on a Hot Tin Roof", "", and "The Prime of Miss Jean Brodie", as well as the film "Picnic" and the ballet "Who Cares?". He won the Academy Award for best color art direction for his work on "Picnic."