Singer was born in 1863 in Yonkers, New York, to Isaac Singer, the founder of the Singer Sewing Machine Company, and his wife Isabella Eugénie Boyer, a French model. He was the couple"s first child, though Isaac had at least eighteen children by a number of previous wives and mistresses. Shortly after Mortimer"s birth, his parents moved from New York to Paris, and then, following the outbreak of the Franco-Prussian War in 1870, to Oldway Mansion in Devon, England.
His father died in 1875 and the children, with their mother, inherited substantial wealth.
Adam was the eldest of Isabella"s children.
He had three brothers and two sisters. Of these, his sister Winnaretta married into French nobility and became a patron of the arts, while his brother Washington was a philanthropist and racehorse owner.
Singer matriculated at Downing College, Cambridge, in October 1881. His youngest brother, Eugene, would later study at the same college.
He left the university without taking a degree.
While originally born an American citizen, he naturalised as a British subject in 1900.