[1][2] He graduated from Ludlow's Black River Academy and taught school while studying law.

[5] A Republican, Slack served as St Johnsbury's Village President, Caledonia County State's Attorney from 1898 to 1900 and a member of the Vermont Senate from 1904 to 1906.

In January 1915, public outcry over the Vermont General Assembly's failure to reappoint longtime justices Loveland Munson and Seneca Haselton led to resignations and new appointments which returned Munson and Haselton to the bench.

As part of this effort, Slack agreed to return to service as a superior court judge.

In 1919, Slack was again appointed an associate justice of the Vermont Supreme Court, and he relocated to Montpelier.