In 1861, he became a partner of Jay Cooke in the banking firm of Jay Cooke & Company, based in Washington, D.C. where he attracted attention "by negotiating for large war loans during the civil war."
 While in Washington, he began his interest in railroads, becoming treasurer of the Washington & Georgetown Line before moving to New York City in 1866 as a member of Jay Cooke, McCulloch & Co., alongside Cooke and Hugh McCulloch (the former Comptroller of the Currency under Lincoln).
 He was with the firm until its collapse during the Panic of 1873.

After Jay Cooke, McCulloch & Co. failed, Fahnestock made an arrangement with John Thompson and his son, Samuel, then president of the First National Bank of New York, by which he took charge of the bond department immediately with the agreement that in 1877, he would be elected vice president and director of the bank and assume control of the bank from the Thompson family (who then founded Chase National Bank in 1877), the same year George F. Baker was elected president.
 Fahnestock led First National Bank, a predecessor to Citigroup, for twenty-five until his death in 1914.
 He also served as a director of the Southern Railway, the Delaware, Lackawanna and Western Railroad, the Central Railroad of New Jersey, the American Cotton Oil Company (a predecessor company to Hellmann's and Best Foods, now part of Unilever), and the Western Union Telegraph Company.

Fahnestock & Co. was founded on May 11, 1881 by his son William Fahnestock, Joseph T. Brown and H. C. Fahnestock as special member.
In 1936, the firm took over the business of H.L. Horton & Co., and eventually led to creation of Oppenheimer & Co. in 1950.
In 1861, he became a partner of Jay Cooke in the banking firm of Jay Cooke & Company, based in Washington, D.C. where he attracted attention "by negotiating for large war loans during the civil war."  While in Washington, he began his interest in railroads, becoming treasurer of the Washington & Georgetown Line before moving to New York City in 1866 as a member of Jay Cooke, McCulloch & Co., alongside Cooke and Hugh McCulloch (the former Comptroller of the Currency under Lincoln).  He was with the firm until its collapse during the Panic of 1873.
After Jay Cooke, McCulloch & Co. failed, Fahnestock made an arrangement with John Thompson and his son, Samuel, then president of the First National Bank of New York, by which he took charge of the bond department immediately with the agreement that in 1877, he would be elected vice president and director of the bank and assume control of the bank from the Thompson family (who then founded Chase National Bank in 1877), the same year George F. Baker was elected president.  Fahnestock led First National Bank, a predecessor to Citigroup, for twenty-five until his death in 1914.  He also served as a director of the Southern Railway, the Delaware, Lackawanna and Western Railroad, the Central Railroad of New Jersey, the American Cotton Oil Company (a predecessor company to Hellmann's and Best Foods, now part of Unilever), and the Western Union Telegraph Company.
Fahnestock & Co. was founded on May 11, 1881 by his son William Fahnestock, Joseph T. Brown and H. C. Fahnestock as special member. In 1936, the firm took over the business of H.L. Horton & Co., and eventually led to creation of Oppenheimer & Co. in 1950.
Harris C. Fahnestock