It took a toy gun costing $1.50, and a pizza costing a bit more, to get Thomas Harrison from the Blue Ridge Mountains of Maryland to the steel-and-glass canyons of Manhattan.

“I still remember overhearing my parents about that toy gun – they didn’t have the money to buy it for me,” said the CEO of Omnicom Group’s Diversified Agency Service, the world’s largest holding group of marketing services companies. “It was well past midnight. I must have been 10 or 11 years old then. I told myself then that I would never allow myself to be in my parents’ situation – not being able to afford a little toy costing $1.50 for their only son, anxiously wondering where the money would come from.”

The incident, as much as anything else, could explain how it was that the son of Elmer Harrison, a school bus driver in the small Maryland town of Cascade, became an executive with 183 agencies from around the world reporting to him.

There was another early incident in Mr. Harrison’s life that may also offer a clue to his ascent. He was barely 15, and earned 25 cents a week cleaning his father’s bus. He had his heart set on acquiring a 1950 Plymouth that was on sale for $175. It meant that he’d have to take extra jobs, including bagging groceries, to save up. Within a few months, he walked up to the seller and gave him 700 quarters. The car was his.

“Here I was, a 15-year-old boy, still a year away from getting a driver’s license – but I had a car,” Mr. Harrison said.

Yet another story suggests how Mr. Harrison built his ambition.

One day, while he was talking with his mother, Phyllis, as she washed dishes, she said to him: “If you don’t become president, you’ll become a millionaire. But you’ve got to work hard. You must become as smart as smart can be.”

She may have said that out of affection, perhaps even to boost young Tom’s morale, but the thought lodged in his mind.

That mind was nourished by a gift from his parents, who’d offered him a choice of a go-cart or 27 volumes of the Encyclopedia Britannica. Tom chose the encyclopedias. He still has them.

The ethos of his early years was captured in an essay by one of Mr. Harrison’s three children, Michael, who was then a 10th-grade student. It was an essay that deeply moved his siblings, Matthew and Lindsay, and certainly his mother Pamela. “But no one was more touched than me,” Mr. Harrison said.

Michael’s essay was titled “From Bag Boy to CEO: An Unconventional Path to Entrepreneurship.”

That path took Mr. Harrison through West Virginia University, where his adviser, Richard Sutter, suggested he abandon his doctoral program in biochemistry and instead crack the marketplace.

“I had been a diligent student, so I was disheartened,” Mr. Harrison said. “But my professor said he was struck by my people skills, and that I should put them to use and earn money.”

Mr. Harrison knew he needed to make money. His monthly salary was $333, while the rent on the apartment he and his wife shared was $255.

One evening they felt like having a pizza. They were so short of money they had to scrounge around for pennies and quarters to pay for their meal.

“That pizza became for me what the toy gun had been for my parents,” Mr. Harrison said. “I have never looked back since.”

He hasn’t needed to look back. Mr. Harrison became a pharmaceutical representative in Maryland at Pfizer, which eventually promoted him and moved him to its Manhattan headquarters. That’s where he was introduced to marketing and worked with health care companies. In short order, he founded his own health care ad agency with a friend, Larry Star.

The agency, Harrison and Star, flourished.

“Not too many CEOs had my background, a combination of scientific knowledge and the experience of having been a pharmaceutical salesman,” Mr. Harrison said. “What set us apart was that we had a real understanding of the science behind the marketing of the health care products. We could craft a message around the brain of any doctor.

“We understood the DNA of the brand, the DNA of the consumer, and the DNA of the manufacturer,” he said.

The agency’s accomplishments came to the attention of the CEO of the Omnicom group, John Wren, who persuaded Mr. Harrison to let him acquire it. That raised his stature, and he eventually become CEO of Diversified Agency Services. Peers praise his collaborative, transformative management style.

And what does he make of his path?

“I was an average student who used smarts, instinct, and craft to get to where I am,” Mr. Harrison said. “Of course, I am exhilarated by the success. I am happy with where my career has taken me, but not satisfied.

“I always search for the new idea, a new issue to address,” Mr. Harrison said.

The pursuit of new enterprises led him to recently write a book, “Instinct: Tapping Your Entrepreneurial DNA To Achieve Your Business Goals.”

“The book does not ask that everyone start their own entrepreneurial venture, but instead do what is genetically right for them,” Mr. Harrison said. “I like to encourage people to think more entrepreneurially even in their current jobs, to become more successful. I want ‘Instinct’ to provide an intuitive roadmap for each individual.”

“No individual is born with a perfect set of genes and a perfect distribution of personality traits,” he said. “We all spend our lives trying to compensate for what we inherited in the genetic lottery. I set out to write the first book of its kind to balance the nature vs. nurture argument. I wanted to show that the real model of success is ‘nature seeks nurture’ – what we were born with seeks out compensation mechanisms instinctively.”

As he savors the publication of his book, Mr. Harrison often thinks of where he was born, of Maryland’s Blue Ridge Mountains, and of his journey to Manhattan.

He often thinks of his parents, too. They did not live to see his successes.

“But you always hope that they’re seeing my life from somewhere else,” he said.