Sidney Mang'ong'o started practicing art at the age of 25. Mang'ong'o moved into a studio at the Kuona Trust in 2011, where he remained for about three years. During this time, a lot of time was spent observing the work of other artists, including participation in the 2012 Wasanii Workshop along with artists such as Ephrem Solomon (Ethiopia), Wanja Kimani (Kenya/ UK), and Dennis Muraguri (Kenya). Mang'ong'o cites Dennis Muraguri and Omosh Kindeh as strong influences during the time he spent at Kuona. Following the move from Kuona, Mang'ong'o, for a time, shared a studio with Kota Otieno in Ngong', before moving his practice to his home in Matasia.
Mang'ong'o's practice was marked by experimentation, working primarily in the mediums of painting and collage, often combining of the two. Much of his work relied on the laborious layering of found imagery on paper and board, using textures, colours, and political jargons to explore notions of socio-political, as well as physical degradation. Mang’ong’o’s work gradually shifted from the figurative and overtly representational towards more formal, geometric abstraction. He however, maintained his deployment of colours in vivid combinations and attentiveness to texture. His final work relied on fields of colour, geometric shapes, and textures exploring forms as they appear in the built environment, and physical structures and networks at a micro and macro level.
His exhibitions included Ziba Ufa (Seal the Crack) at the Kuona Trust (2011); X-tract, Subtract, Abstract, Circle Art Agency (2013); Paper II, Circle Art Agency (2015); and Freedom | Flight | Refuge, Circle Art Agency (2016). In 2017, Mang'ong'o was one of four artists presented by Circle Art agency at the Jo'burg Art Fair. In May 2018, Sidney Mang’ongo had a critically acclaimed and commercially successful solo exhibition, Imagined Structures at Circle Art Gallery.