This is a very interesting topic since Marilyn's superstardom did not come overnight, in fact it took her good 6 years of work in many movies before Niagara catapulted her to the top. I'm interested in seeing more of these, pre 1953 MM movies since it's always fascinating to see an icon playing a supporting part of just appearing as a random extra. Looking through her filmography, I realize that I have seen quite a few of her early films so here's a short review.
The Asphalt Jungle (1950), from MGM, is of course her first truly noticeable performance and her first major movie. It was a great chance to do some acting, for the first time, even though the role seems like a typical, blonde bimbo that would go on to haunt her, but MM did solid work here and got some good reviews. The movie itself is one of my favorite crime pictures, a masterpiece that's still equally interesting, exciting and tragic today.
All About Eve (1950), from FOX, is another great supporting part for Marilyn, proving that 1950 is indeed her breakthrough year. Playing a wannabe actress trying to achieve fandom at any cost, must have felt a bit too familiar to her at this point but still it was a great chance to work with industry's finest. She showed some early comedy talent as well, setting up the ground for her future trademark dumb blond character. I wonder if she was intimidated by Bette on set, especially considering her confidence issues. The party staircase scene is one of my all time favorites so just being part of that moment must have been quite an experience for MM.
As Young As You Feel (1951), from FOX, is a funny little comedy that I've seen several times and always enjoyed it a lot. It has a great comedic cast with Monty Woolley and Thelma Ritter at the front, along with other FOX regulars like Jean Peters and David Wayne. Marilyn has a small role as a secretary and does the best she can with it.
Love Nest (1951), from FOX, is a charming movie that I've seen once and loved it. It stars June Haver and William Lundigan as a newly-wed couple moving into a new apartment and dealing with many problems they encounter there. One of those problems is a blond bombshell moving into the building and causing a commotion. This role feels like preparation for what Marilyn will do later in Seven Year Itch. I really want to rewatch Love Nest.
Clash By Night (1952), from RKO, is a noir melodrama that I always enjoy watching. Barbara Stanwyck is great as usual and delivers some inspiring lines as a bitter, washed out woman returning to her small town and dealing with all sorts of issues. MM has quite a few scenes, playing Barbara's brother's girlfriend, sharing many scenes with Stanwyck and having great banter with her. I love their scenes together, Stanwyck was known as a good colleague and I'm sure MM enjoyed working with her and learning from her.
Don't Bother To Knock (1952), from FOX, is Marilyn's first starring role and a unique film in her opus. Some in fact think it's her best work, playing a mentally unstable nanny. She goes back to brunette for this one, the last time in fact.
Monkey Business (1952), from FOX, MM's last film before she's off to superstardom. A wacky, screwball comedy with a great cast, including Cary Grant, Ginger Rogers and Charles Coburn is always an enjoyable watch. Marilyn plays another secretary but has a good amount of screen time, sharing a crazy drive with Cary, being chased around by man acting as children, having to deal with crazy monkeys and all kinds of shenanigans.