Alexander Cosby (c 1685–1742) was born in Ireland and had a younger sister who married Richard Philipps, governor of Nova Scotia and a military man. Through this connection, Cosby was appointed major in the 40th regiment and was stationed in Nova Scotia around 1721. He looked to be appointed lieutenant governor in 1725 but the appointment went to Lawrence Armstrong.
This was a time when the governor was often absent as was the case with Philipps. Cosby refused to follow orders given by Armstrong and also was appointed lieutenant governor of Annapolis Royal in 1727 by Philipps to succeed John Doucett. It appears that Cosby was always in some kind of turmoil within the military and the governing body, a condition which continued until his death. Accounts of his actions must be tempered by the fact that his correspondence did not survive and accounts are only from the other perspectives.