After his army days, Slaughter returned to cattle and ranch life, and having an eye for business, he purchased 246,699 acres and established the Lazy S Ranch. He continued buying up land across West Texas, and by 1906, the Cattle King of Texas owned 40,000 cattle and over one million acres of ranch land.
Slaughter was a devout Baptist and contributed the majority of the cost for the construction of the First Baptist Church in Dallas and his generous contributions also allowed the establishment of the Texas Baptist Memorial Sanitarium, now Baylor Hospital.