Isaac Seligman was born in Baiersdorf, Kingdom of Bavaria. He was the youngest of eight brothers, all of whom emigrated to America and became involved in running various branch offices of the merchant banking house J. & W. Seligman & Co., co-founded in Manhattan, New York City in 1846 by Isaac's elder brothers, James and Joseph Seligman. Isaac went on to run Seligman Brothers, the London branch of the Seligman merchant-banking empire with his brother Leopold. He was also a fundraiser for, benefactor to, and activist in, a large number of charitable and political organisations.