Tracy Britt, 28, has become one of the most influential women within Warren Buffetts Berkshire Hathaway. Anupreeta Das reports.

The seed Ms. Britt planted that day yielded quick results: a job for Ms. Britt as Mr. Buffett’s financial assistant at Berkshire Hathaway Inc. Almost four years later, it has blossomed further, with Ms. Britt emerging as one of Mr. Buffett’s top lieutenants and even serving as chairman of four companies within his $284 billion conglomerate.

The firms in which she serves as chairman, including building-products company Johns Manville Corp. and paint manufacturer Benjamin Moore & Co. total more than $4 billion in annual sales. In March, a few weeks after Berkshire and Brazilian investment firm 3G Capital said they would buy ketchup maker H.J. Heinz & Co. for $23 billion, Mr. Buffett sent Ms. Britt to Brazil, according to people familiar with the matter.

The deal was Berkshire’s largest acquisition since 2010, and Mr. Buffett wanted her to know more about 3G’s operations, including how the Brazilian firm had turned around Burger King Worldwide Inc. the people said.

Ms. Britt is one of the executives the 82-year-old Mr. Buffett is grooming for senior positions after he steps down, say people familiar with the matter and Berkshire analysts. And she isn’t the first person that he picked out of relative obscurity: His investment managers, Todd Combs and Ted Weschler. were little-known hedge-fund managers before Mr. Buffett tapped them to handle big slices of Berkshire money.

Ms. Britt is also one of the most influential women within Berkshire, which has three women directors on a 13-member board and five women CEOs out of 81 operating companies.

Ms. Britt takes care of all kinds of things that come up, Mr. Buffett told college students in Omaha last month.

Ms. Britt and Mr. Buffett declined to comment on her role at the company.

Others familiar with the company say Ms. Britt is increasingly serving as a conduit between Mr. Buffett and the CEOs of Berkshire’s various businesses.

Although Mr. Buffett is famously hands-off when it comes to running Berkshire’s far-flung units, some CEOs had requested more interaction with each other, wanting to tap other executives as a resource to learn about management practices and other potential ways to collaborate, according to people familiar with those discussions. Ms. Britt facilitated some of those discussions and arranged Berkshire’s first-ever CEO roundtable on the sidelines of the annual meeting in May.

She’s got this wealth of information about what’s going on at the Berkshire portfolio companies, said Sam Taylor, chief executive of Oriental Trading Co. an online retailer that Berkshire bought in November. She knows exactly what’s going on and where Warren’s head is at.

Mr. Taylor, 52 years old, said Ms. Britt is wise beyond her years.

Others say she always has stood out among her peers.

Janet Hanson, the founder of 85 Broads, a global networking community for women in business and finance, remembers Ms. Britt from when she interned at Ms. Hanson’s investment firm as an undergraduate. She had some sort of internal guiding system that was propelling her forward, Ms. Hanson said. She was on rocket fuel…this was a gal who was going to get what she wanted.

That summer, Ms. Hanson asked her interns to pick stocks they knew as consumers and thought were good investments. While some of her peers picked highfliers like BlackBerry maker Research In Motion Ltd. among Ms. Britt’s picks was Anglo-Dutch consumer company Unilever PLC, the maker of Dove skin-care products and Lipton tea, the type of nuts-and-bolts company that Mr. Buffett has acquired at Berkshire.

Ms. Britt grew up on a family farm—Britt’s Garden Acres, outside of Manhattan, Kan.—that produces more than 40 different types of fruits and vegetables ranging from radishes to rhubarb.

She first met Mr. Buffett in 2006 after she arranged a trip to Omaha with Harvard classmates from an organization she co-founded, called Smart Woman Securities, which helps women learn about investing.

Some analysts and Berkshire observers said they see Ms. Britt becoming a top Berkshire executive in the next decade, potentially performing managerial functions such as fixing troubled businesses or negotiating deals, similar to what David Sokol previously did.

Mr. Sokol is a former Berkshire business manager who was considered a protégé of Mr. Buffett’s. He resigned in 2011 after disclosures that he had personally bought shares in chemicals company Lubrizol Corp. not long before recommending that Mr. Buffett buy it. Regulators investigated but decided this year not to take any action against Mr. Sokol. Mr. Sokol’s lawyer has said that his client didn’t violate the law or any Berkshire policy at any time, or intend to personally profit at the expense of his former employer or its shareholders.

She’s stepped into the role of Sokol, said Robert Miles, who teaches a course on Mr. Buffett’s investing style and has written three books on Berkshire.

For Ms. Britt, none of this is a complete surprise.

In a recent blog post for Lean In, an online community created by Facebook Inc. Chief Operating Officer Sheryl Sandberg as part of her campaign to get women to pursue their ambitions, Ms. Britt shared a line from a business-school assignment she had penned to imagine where she would be at the 10-year reunion. She wrote, My goal is to work with a great investor, who even more importantly is a wonderful teacher and mentor.