As perspectives upon accountability are changing, researchers are making efforts to redefine conversations about how teacher quality enhances the quality of education and gains in students' achievement. Mid-career teachers, who are a school's greatest source of professional capital and expertise, are often neglected in these discussions, and may become uninterested over time, with their performance levels dropping. As the experiences of mid-career teachers can only evolve with exploration and implementation of new ideas, the aim of our research is to identify the evidence-informed indicators of mid-career teacher quality. Our article intentionally focuses on the research design and methodology, also known as the 'study protocol', that aims to present a research design before the actual research begins. Our study protocol will use a sequential mixed-methods design. A systematic review of literature will be conducted followed by semi-structured interviews with stakeholders to identify the indicators of mid-career teacher quality. Lastly, an integrated analysis of two data sets will be conducted to determine which of the indicators, as extracted from the systematic review, resonate with participants' perspectives, thus providing a holistic approach to understand mid-career teacher quality. The study is anticipated to inform policy and other initiatives that can strengthen the mid-career teacher workforce.