Count Losy's family were of Swiss origin (Poschiavo in the canton of Graubünden). His father, Johann Anton Losy senior (c. 1600–1682), was perhaps born in Purz in the Swiss canton of Grisons. By 1627 he had moved to Bohemia and purchased a house in Prague. Losy senior had a number of business interests and was appointed a Councillor of the Exchequer and Deputy of the Salt, Beer and Wine Council by the Bohemian Court Chamber. As a result of helping to defend Prague against the assault of Swedish troops in 1648, Losy senior was ennobled, becoming a baronet in 1648 and Count von Losinthal in 1655. He also acquired the estate and castle at Steken (Strakonitz District, southern Bohemia) in 1638.