Mullis founded Sentry in 1973 in Columbia, South Carolina, not as an electric-fence company, but as a guard dog business. For more than two decades, he supplied trained guard dogs to industrial and commercial sites, like auto yards, scrap dealers, and logistics terminals: businesses whose most valuable assets often sat outdoors and were expensive to secure. But in 1991, thieves repeatedly broke into Mullis’s own dog yard. For a man selling security, it was not a good look.

So, he built his own solution.

Mullis drilled holes into tall PVC poles, threaded high-tensile wire through them, and electrified the inner perimeter with a psychologically intimidating—but medically safe—7,000 volts. The results were immediate and absolute: the thefts stopped. Word spread quickly. Visitors to the dog yard wanted to know what the strange inner fence was, and whether he would build them one.

At first, Mullis refused. He was in the guard dog business, after all. But the requests kept coming. Finally, in an attempt to end the conversation, he told one particularly persistent customer that he would not sell them a fence, but he would lease them one…at a monthly rate so absurdly high he was sure they would decline.

They accepted on the spot.

That moment launched what would become one of the most quietly successful non-software recurring-revenue businesses in the country, long before “X-as-a-Service” entered the business lexicon. Mullis realized he was not just selling a fence. He was selling deterrence, peace of mind, and a mission-critical outcome. Leasing the fence eliminated customer capex barriers, ensured consistent maintenance, and created a sticky, high-margin subscription revenue stream. Over time, the electric-fence business did not just supplement the guard dog business—it replaced it entirely.

Demand accelerated across industries with high-value outdoor assets: auto auctions and dealerships, salvage and dismantling yards, metal recyclers, building materials distributors, equipment rental depots, trucking yards, rail centers, ports, warehouses, and municipal and infrastructure sites. The value proposition was universal: stop theft, avoid capital outlays, and let someone else maintain the system.

By 2007, Mullis and his two adult children had built a sizable and enviably predictable business, still rooted in Bill’s hands-on ethos. He was known to crisscross the country with a trailer of fence parts, fixing and upgrading systems personally. That year, he sold a controlling interest to Ulysses Management, a New York–based family office/private equity firm. Under institutional ownership and with professionalized operations, the business more than tripled. It was eventually sold to Snow Phipps Group (now TruArc Partners), which extended the model further through add-on acquisitions, enhanced monitoring technology, integrated lighting solutions, and third-party security-system interfaces.

In 2020, the Company rebranded as AMAROK, a name referencing a mythological wolf, while continuing to market its iconic Electric Guard Dog™ as its flagship product. A year later, the business changed hands again and is now majority-owned by Northleaf Capital Partners and AVALT, with continued growth supported by the strength and predictability of its long-term contracted revenue base.

Like many enduring businesses, the story began with a founder solving his own problem. Mullis became his own first customer, and in doing so, uncovered a deeper truth: the real innovation was not just the fence, but the business model behind it. By renting security rather than selling equipment, he removed friction from the buying decision, delivered consistent outcomes, and built a durable recurring revenue platform in the industrial and commercial security market.

Part accident, part foresight, and entirely effective, Mullis’s insight transformed a local guard dog yard into a national infrastructure security powerhouse. Investors spend considerable time searching for businesses just like this: sticky, subscription-based, and serving mission-critical needs. Bill Mullis found and built one by simply working to solve his own problem and packaging it to others as renting them a consistent outcome rather than selling them a piece of equipment.