Mackereth was born on January 27, 1958, in Washington, D.C.. She graduated from Montgomery Blair High School in 1975 and earned a Bachelor of Science degree in psychology and sociology from Frostburg State University in 1979.
[4] In 1980, Mackereth became a caseworker for York County, Pennsylvania's Children and Youth Services, becoming a supervisor by 1987.
She was continuously re-elected until her retirement in 2008 when she announced her appointment as York County's human services director.
[5][7] Mackereth served as Deputy Pennsylvania Secretary of Public Welfare for the Office of Children, Youth and Families from 2011 until 2013, when Governor Tom Corbett appointed Mackereth acting secretary of public welfare, following the resignation of Gary Alexander on February 15, 2013.
[10] In 2024, Mackereth was hired by Lackawanna County as a consultant charged with reviewing the policies of the county's Office of Youth and Family Services after the state government downgraded the agency's rating following the arrest of several caseworkers on charges of child endangerment.