Mun Sen worked as a retail assistant at a bookshop and also worked as a cashier at a school bookshop. In the 1955 Poay Cheng Building School Fund catalogue, it is recorded that he was paid fourteen dollars (presumably a month?) for working at the school, 6 dollars went to his rental. The income wasn't enough to support his growing family. He painted in his free time and supplemented his income by selling pictures he painted at a relative's book store in Singapore, Rui Ting 瑞廷書局. Joseph McNally (1994) lists the bookstore as Kee Huar Bookstore.

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1972. Pameran Peringatan Yong Mun Sen/ Yong Mun Sen Memorial Exhibition. Exhibition catalogue 17 March - 4 April. Kuala Lumpur: Balai Seni Lukis Negara/National Art Gallery; 金戈 Jing Ge，1955。’楊曼生傳 (Biography of Yong Mun Sen)‘ Poay Cheng Building School Fund Exhibition Catalogue, 4; Joseph McNally, 1994. 'Yong Mun Sen' Pioneers of Malaysian Art, 70-76.