[5] Slack originally enlisted in the Illinois Air National Guard in 1959.
He would transfer to the Wisconsin Air National Guard in 1961 and was commissioned a second lieutenant.
He was promoted to Major General of August 3, 1990 and his retirement was effective as of February 13, 1996, the day before his 60th birthday.
[6][7][8] Awards he had received include the Legion of Merit, the Meritorious Service Medal, the Air Force Commendation Medal, the Army Commendation Medal, the Air Force Achievement Medal, the Air Force Outstanding Unit Award with oak leaf cluster, the Air Force Organizational Excellence Award, the Combat Readiness Medal, the National Defense Service Medal with service star, the Air Force Longevity Service Award with silver oak leaf cluster and two bronze oak leaf clusters, the Armed Forces Reserve Medal with two hourglass devices, the Small Arms Expert Marksmanship Ribbon with service star, and the Air Force Training Ribbon.
In 2019, he was inducted into the Wisconsin Air National Guard Hall of Fame.