For a more detailed account of his investment career check out the book THE HELMSLEYS. For a shorter version read the following link from American National Biography Online: http://www.anb.org/articles/10/10-02292.html
Here's part of the bio from the above link. Note how he was patient and bought undervalued assets during financial turbulence. This is a common think among almost all billionaire investors.
"Helmsley, Harry (4 Mar. 1909-4 Jan. 1997), real estate magnate, was born Henry Brakman Helmsley, the son of Henry Helmsley, a wholesale dry goods buyer, and the former Minnie Brakmann. The family moved from Manhattan to the Bronx, New York, several years after his birth. Harry, as he was nicknamed, and his younger brother, Walter, were educated in local public schools. Harry graduated at the age of sixteen from Evander Childs High School in the Bronx but could not afford to attend college full time; instead, he went to work to help support his family. He was hired as an office boy by a city real estate firm, Dwight, Voorhis and Perry, which paid him twelve dollars a week. Harry had secured the job through the connections of his maternal grandfather, who had been a city landlord, and he quickly proved adept at the business. Soon he was assigned to rent collection from properties managed by the firm; eventually he was given properties to oversee himself. His salary rose accordingly, as did his knowledge of buildings throughout New York City. During this time he also began a continuing program of self-education, taking extension classes at Columbia and New York universities as well as the YMCA. His favorite subject was history.
A Knack for Selling Helmsley's career in real estate had begun during the financial boom of the 1920s, and it continued in full swing even as the nation sank into a major economic depression following the Wall Street crash of 1929. By the early 1930s he had become a proficient broker, with a particular aptitude for placing accurate market valuations on buildings and finding prospective buyers. He also began to dabble in real estate himself, making his first purchase, a ten-story office building on East Twenty-third Street, for a thousand dollars in 1936 from an owner who was about to default on his mortgage. Helmsley hired his unemployed father as the building manager. That same year he was named secretary of the company, and in 1938 he became a partner; henceforth, the company was known as Dwight, Voorhis and Helmsley. Soon afterward, Helmsley hired his brother, Walter, as an employee. That same year he married Eve Ella Sherpick Green, a widow.
Buying Spree As the Great Depression worsened, Helmsley bought dozens of properties at bargain prices. His buying spree continued as the Depression gave way to World War II, and during the 1940s he rose to become head of the firm, now renamed Dwight-Helmsley. Assisting him with his real estate investments was Lawrence A. Wien, a like-minded lawyer with similar skills. Helmsley later added two other important associates, Alvin Schwartz and Irving Schneider, to his inner circle.
Helmsley continued to prosper during the burgeoning postwar economy. By the early 1950s he had emerged as a major investor in New York real estate, although the majority of his holdings were relatively small buildings in less desirable areas of the city. Then, in 1954, Helmsley made a major acquisition: the Lincoln Building, a prestigious office skyscraper across from Grand Central Station on Forty-second Street in Manhattan. A year later, he bought a real estate management company owned by Leon Spear, and the firm became known as Helmsley-Spear. Helmsley's rise continued unabated, driven by his keen understanding of economics and the real estate market. His basic strategy involved the securing of long-term fixed-rate mortgages during periods of inflation; when interest rates fell, he reduced his indebtedness and paid with cash. This was not a common strategy at the time, but it is now considered standard business practice among real estate investors.
In 1961 Helmsley bought New York's Empire State Building, then the tallest building in the world, for $65 million; he was now arguably the most influential real estate magnate in the city. At the time, however, the sale was considered a somewhat dubious transaction because of the building's notoriously high operating costs. By pairing cost-cutting measures with rent increases, Helmsley was able to turn his investment into a profitable undertaking. In 1965 Helmsley made significant further additions to his real estate empire with the purchase of two New York brokerage houses, Charles F. Noyes Company and Brown, Harris, Stevens. Further expansion occurred four years later with his acquisition of more than two dozen New York buildings from a city real estate trust owned by Morris Furman and Herman Wolfson. In the following decades, Helmsley's empire grew to include such New York commercial landmarks as the Graybar, Fisk, and Flatiron buildings as well as major residential developments, including Manhattan's Tudor City and Park West Village.
By the late 1960s Helmsley had also launched a parallel career as a real estate developer. His projects eventually included the erection of several notable skyscrapers in New York City, including the Marine Midland Bank building in lower Manhattan, the Pfizer building, and One Penn Plaza; he also built the Merchandise Mart in Los Angeles. His real estate acquisitions had expanded to other cities as well: by the early 1970s Helmsley owned buildings not only in New York City but also in Philadelphia; Washington, D.C.; St. Louis; Chicago; Houston; Los Angeles; and San Francisco and in smaller cities across the country. Over the years, he established some four hundred holding companies to manage his properties. In New York City alone, their estimated value by the 1980s was placed at $2.5 billion.
Lavish Lifestyle In 1972 Helmsley divorced his wife of thirty-four years and married Leona Roberts, a divorcée and onetime model turned real estate broker who had been hired by Helmsley as a sales executive at the Helmsley subsidiary Brown, Harris, Stevens and was twelve years his junior. During his first marriage, Helmsley had by all accounts led a quiet social life, centered on the couple's home in the Gramercy Park neighborhood of Manhattan's East Side. Following his union with the extroverted Leona Roberts, he became more outgoing and began to pursue an opulent lifestyle, based in a penthouse at the Helmsley-owned Park Lane Hotel on Manhattan's Central Park South. A highlight of their social life was Helmsley's annual birthday celebration, billed as the "I'm Just Wild about Harry" party, organized and hosted by his wife at the Park Lane Hotel. Harry reciprocated by lighting the Empire State Building red, white, and blue every July Fourth, to honor his wife's birthday. In addition to their New York penthouse, the couple established and maintained residences in Palm Beach, Florida, and Greenwich, Connecticut, and later had another home in Paradise Valley, Arizona. In 1973, a year after their marriage, both Helmsley and his wife were seriously injured during a robbery of their Palm Beach home, but they subsequently recovered."