[1][2] His father, one of Philadelphia's wealthiest men by the 1890s,[3] was an accountant and manufacturer, who created a large chemical establishment and had an interest in numerous other profitable business ventures, including the Pennsylvania Railroad.

[2][4][5] Rosengarten received his early education in private schools in Philadelphia, and for a time came under the influence of a scholarly man in York, Pennsylvania, the Rev.

[4] After graduation he studied law in the offices of Henry M. Phillips, a prominent member of the Philadelphia Bar, and in 1856, Rosengarten was admitted to practice.

In pursuance of this plan Joseph Rosengarten went...to study history and Roman Law at the University of Heidelberg and to engage in travel.

[4]While abroad he was "thrown into contact with men of distinction," including Ludwig Häusser and de:Adolph von Vangerow, who served respectively as professors of history and law at the University of Heidelberg.

[4] His train stopped short of Harpers Ferry, he walked into town, and witnessed the attack made by Lee's men on the engine house where John Brown had sought refuge.

Rosengarten fought with distinction at the Battle of Fredericksburg, "distinguishing himself for bravery, picking up the colors after four sergeants had been disabled and carrying them successfully through the engagement.

[4] Rosengarten maintained a lifelong interest in his alma mater the University of Pennsylvania, serving as president of the alumni association, and as a member of the board of trustees.

[4] At his urging, the institution was moved from Philadelphia to the outlying countryside, where, instead of being "treated as prisoners the boys and girls were placed in homes organized on the cottage system," provided with educational opportunities, plenty of fresh air, outdoor work and exercise.

[4] He also delivered numerous addresses before various literary and charitable associations, including one before the Historical Society of Pennsylvania on the "Life and Public Services of General John F. Reynolds" (Philadelphia, 1880).

[4] He contributed translations of Hessian soldier Stephan Popp's Revolutionary War diary and Achenwall's Observations on North America to a paper, "American History from German Archives," published in the Society's Proceedings for 1900.

[1][4] In 1904, Rosengarten was awarded the Cross of the Legion of Honour by the Republic of France for helping to foster American interest in French culture.

He knew the Darwins, father and son; he came into close touch with eminent writers and scholars like F. Max Müller, Thomas Hughes, Goldwin Smith, Herbert Spencer and Lord Bryce; he formed a friendship extending over many years with the de Rochambeau family and secured the passage of an act of Congress for the purchase of the letters of Washington to [the Comte de] Rochambeau.