Kiema et. al., (2015) found that some construction companies within Mombasa County do not accord the internal auditors the independence they deserved to effectively give their opinion on the financial statements of the organizations. The study did not give a clear direction of the relationship between internal audit function as a whole with the financial performance of firms in the mid- size manufacturing sector. Ondieki (2014) conducted a survey of the effects of internal audit on financial performance of commercial banks in Kenya and found that internal audit standards, independence of internal audit, professional competency and internal control had a positive relationship with financial performance of commercial banks. Ondieki looked at internal audit in the context of banking firms and did not link the internal audit function to the financial performance of the banks and therefore could be interesting to link the internal audit function and financial performance in the mid- size manufacturing firms. As evidenced from the foregoing studies, there is unanimous agreement that if left unexploited internal audit is most likely to affect the financial performance of mid- size manufacturing firms. There have been no published studies on the relationship between the internal audit function and financial performance of mid- size manufacturing sector organizations in Kenya hence the research gap. To this end the study sought to respond to the following research question: What is the relationship between the internal audit function and financial performance of mid-size manufacturing firms in Nairobi County? The agency theory outlines the relationship or the dependency between an agent and a principal (Adams, 1994). The principal delegates responsibilities to the agent most often for a fee. It can also be postulated to mean the practice by which productive resources owned by one person or group are managed by another person or group of persons (Millichamp & Taylor, 2008). The agency theory is said to be one on the internal auditing theories. In the context of an organization, the agency theory argues that agents should act in the interest of their employers (shareholders). However, the aforementioned agents have been alleged to act in their own interest rather than on the behalf of the shareholders.>