Over the course of his career, he had three wives, and rose from being simply a member of the mounted guard, to standard-bearer, to captain of a unit of the provincial defense forces, to captain of a unit of twenty gunmen, to captain of one unit of the domain's mounted guard.
In addition to his military/government service, Hirosada is known to have pursued an interest in history, and certain scholarly and cultural practices. He practiced tea ceremony, and studied and hand-copied a number of documents of a variety of sorts, from records of court ceremonies to war tales, and even cookbooks. He maintained a diary from age 22 until age 63, the year before his death; this has become a valuable resource for historians for understanding some of the goings-on of the domain during that period. Hirosada also led a group of samurai in reading through various documents, sometimes with a particular focus on proper samurai behavior.
Hirosada's first two wives were both of samurai status, and he had one daughter, Otsune, by one of them. He also had a daughter named Omase, with one of the household servants, a woman named Riso. Following the death of both of his samurai wives, in 1764, at age 54, Hirosada married one of his household servants, a 25-year-old woman by the name of Umeno. Though many of Hirosada's friends and family accepted her as his wife, the domain government refused to officially recognize the marriage, Umeno being below samurai status.
Hirosada and Umeno had a son, Mori Yoshiki, in 1768, who would go on to become a fairly successful government official. Yoshiki was Hirosada's first son, but he was not, at least initially, situated to serve as Hirosada's heir, since some years earlier, in 1759, after falling briefly but seriously ill, Hirosada had adopted his nephew, Mori Hirotake, naming Hirotake his heir. Hirosada died in 1773, and was succeeded as family head by Hirotake.