After 6 1/2 years as Burlington’s director of public health, and eight years overall with the town, Sharon Walker Mastenbrook retired last week. A Maynard resident, Mastenbrook was the only person to hold the health director’s position since it was created in October 2005, when the offices of health agent, environmental engineer, and public health nurse were consolidated. Mastenbrook started working as the health agent in 2004. “It’s been an honor to work here,’’ she said before her official departure Friday. “I’ve enjoyed my time. I love public health. I’m really dedicated to the profession. But a new chapter is opening in my life and it’s time for me to move on.’’ Regarding her plans, Mastenbrook said, “I’m going to be a grandmother in September, and I’m looking forward to spending time with my family.’’ The Board of Health will be initiating a search for a successor to Mastenbrook; in the meantime, Susan Lumenello, the town’s environmental engineer, is serving as acting director.