He studied at the Art Students League and the Pennsylvania Academy of the Fine Arts.
He was described as "the most successful set designer of the Golden era of Broadway", and worked on both stage plays and musicals. Mielziner was the brother of actor-director Kenneth MacKenna. Mielziner was considered one of the most influential theatre designers of the 20th century, designing the scenery and often the lighting for more than 200 productions, many of which became American classics.
He "pioneered "selective realism" in scenic design".
According to his obituary, he was perhaps "praised most often..for his sweeping canvas of people under the Brooklyn Bridge, used as a backdrop for Maxwell Anderson"s Winterset. 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, Another Participant 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, Gypsy, and The Prime of Mission Jean Brodie, as well as the film Picnic and the ballet Who Cares?.
During World World War II, Mielziner worked as a camouflage specialist with the United States Air Force.
His influence extended outside of the theatre. He was acquainted with the American artist Edward Hopper, who is said to have modeled his well-known painting Early Sunday Morning after Mielziner"s set for Elmer Rice"s play Street Scene, produced in 1929. Mielziner designed the theater at Wake Forest University and co-designed the Vivian Beaumont Theater at Lincoln Center with architect Eero Saarinen.
He resided for many years at The Dakota and can be seen working in his studio in an exterior shot in the film Rosemary"s Baby.
He died on March 15, 1976, four days before his 75th birthday, in New York City. Director Mike Nichols stated that he felt he needed Mielziner"s original set because it was "intimately connected with the way the play developed." He went on to say he has never seen anything "near as good in any of the productions of "Salesman" because it"s everything and nothing.".