Expressing breast milk is one of the recommended approaches for working mothers. However, its adoption has been low. This study aimed to establish the constraints of expressing breast milk among lactating mothers. A cross-sectional study design was adopted on a comprehensive sample of 85 working lactating mothers in Nairobi County, Kenya. Most of the working mothers (67.1%) had a maternity leave of three months. Only 18.9% expressed breast milk, citing they understood its importance. Of these only 7.1% had breast pumps while 11.8% were hand expressing. The main reasons to this were; lack of knowledge on importance of breast feeding (12.2%), lack of knowledge on how to express (20.1%), lack of breast pumps and fridge (13.1%), lack of time to express (9.6%), poor perception of the community on the practice (8.3%), fear of contamination (7.9%) and suspicion that the house helps/caretakers will not feed the child with the expressed milk (7.0%). The study concludes that most working mothers do not express milk due to lack of knowledge on its importance and how to express milk, availability of equipment and time. This study recommends creating more awareness on importance of expressing breast milk.