Known as Justina to her friends and relations, Brandly Lazarus, onlychild of Frumet Cohen and Sampson Lazarus, was likely born inFredericktown, Maryland, where her father, uncle, and bothgrandfathers were well established in trade. However, the family hadlimited communal support in such an isolated spot. Brandly was 27 andstill unmarried when her uncle Isaac, who had been her father’sbusiness partner, died childless in April 1779. A year later, SampsonLazarus moved his family to Lancaster, Pennsylvania, hoping toimprove his daughter’s prospects for marriage. The strategy paidoff, and in 1781 Brandly was married to 36-year-old JoshuaIsaacs. Jr., then a Pennsylvaniamilitiaman serving the cause of American independence. Their firsttwo children, Joseph and Frances, wereborn in Lancaster, however shortly after the end of the war, theyoung couple moved to New York, where their four youngerchildren—Solomon, Grace, Sampson andHannah—were born.

Brandly seems to have been adored by her children: Frances named oneof her children for her, and son Solomon named two. Brandly wouldsurvive her husband by 15 years, died on the anniversary of hisdeath, and was buried beside by him in the Chatham Square cemetery inNew York City.