Although she has spent most of her career as a practicing primary care physician, Mullen has turned increasingly toward public health and policy work in recent years. Before Gov. Dannel P. Malloy selected her to lead the Connecticut Department of Public Health, Mullen spent two years as director of the Bureau of Community Health and Prevention at the Massachusetts Department of Public Health. Her first day on the job coincided with significant cuts to the department's budget, giving Mullen experience dealing with dwindling resources that could prove useful in her new job. Mullen has served on the medical staff at the Hospital of St. Raphael in New Haven, the Yale University Health Services and Yale-New Haven Hospital. She was named "outstanding provider of the year" by Yale University Health Services in 1998. In 2001, she became medical director of the Baystate Mason Square Neighborhood Health Center in Springfield, Mass., a job she held until she was appointed to state government. Mullen is an advocate for efforts to prevent, not just manage, chronic diseases, and has said she wants to address health disparities. Connecticut as a whole tends to perform well on many health indicators, but that can obscure significantly higher rates of diabetes, obesity and other conditions among minorities. Mullen's husband, Dr. Herbert Knight, is chief of pulmonary and critical care medicine at the Hospital of St. Raphael. They live in Guilford and have two children.