Around the same time the Ince studio was running a beauty contest and Marion also entered that too by sending them her picture. She received the standard "we'll let you know" reply so naturally thought nothing more of it. Marion interviewed with Jack Waldron, Sennett's manager and was hired by them for $25 a week. It was about a week after this that her picture appeared in the paper with an announcement that she had won the Ince contest, now they wanted her too! Marion had to tell Hans Sternberg, Ince's publicist that she already had a job with Sennett which did not please him, as Marion said "Ince's publicity man got a little mad at me". Marion didn't want to work for Ince as she already had the Sennett job, and in those days Sennett was the king. Sternberg insisted though that she would have to pose for some Ince publicity at the Billion Dollar Theater, which she agreed to do. Sternberg showed the photos to a fellow worker for Ince, Louis Lewyn who took one look at them and said "What a lemon, however did you pick her?"

Sometime later Lewyn was over at the Sennett lot taking pictures of some of the girls, suddenly he saw Marion and either liked the look of her in make up or the Ince photos had been really bad. He asked her if she would pose separately from the other girls and then kept asking her if he could bring her home. She informed him that her father wouldn't let her date but he kept on insisting so she let him take her to her house. On the way there he asked her how she got in the movies. Marion told Lou about the Ince contest and how she had won it. Lou nearly fainted! That is when Marion learned the whole story concerning Lou's first thoughts about her. They had apparently changed permanently and Marion must have forgiven him, because they married and remained married until his death in 1971.

Marion Mack said of Mack Sennett that she was a little scared of him at first because he sounded a little rough but he apparently liked her. He gave her the screen name of Joanne McGuire. Marion became bored with just standing around and being filmed in a bathing suit, she wanted to act so she took the first good offer that came along. She left Sennett and joined the Little Mermaid and Sunshine Comedies who asked to change her name, she decided to use the middle part of her given name and became Marion Mack. By 1922 she was making feature length films and in 1923 she married Lou Lewyn, he was later to become a successful movie producer, from the start he let Marion help write some of the scripts for his films. Together they made 'Mary of the Movies' in 1923 and 'Carnival Girl' in 1926.

Marion's most famous role was as Annabelle Lee in Buster Keaton's movie 'The General'. Keaton was looking for someone to play his leading lady in the civil war drama and he wanted her to look the part with long curly hair. Norma Talmadge was having her makeup done for her then current movie by make up artist Percy Westmore and she happened to mention this to her. Norma Talmadge was aware of Keaton's need because her sister Natalie who was Keaton's wife had told her. Percy Westmore had also done Marion's make up for 'Carnival Girl' and he remembered that she had the right look. Norma asked Percy if Marion was available and he promised to find out. The first thing he asked Marion on the phone was, had she still got her long curls. She had not, she had had her hair cut a few days earlier into a fashionable bob. Percy told her not to worry, he would come up with something, and he did, a wig. Marion went to her interview with Keaton and Lou Anger, Keaton's studio manager, Keaton said "She'll do" and she was promptly hired. 'The General' was Marion's last movie, after that she retired from the screen. She did come out of retirement for a while in late '72 to promote the newly restored 'General'.