Bartholemew Mosse was born in Dysart, 2 km east of Portlaoise (then called Maryborough), the fifth son of seven children born to William Mosse, a Protestant clergyman, and Unknown Miss Boyle. Bartholomew apprenticed with Dr. John Stone as a barber-surgeon from 1729-1733, when he passed examination by the surgeon-general. He married Mary Elizabeth Mallory in 1734, and they had a son, Michael, who was born in 1737. While historical records are unclear, it appears that both mother and infant died shortly thereafter.
In 1738, he was appointed surgeon in charge of the draft of troops to Menorca, following which he spent time traveling through Europe. It is believed that it was during this time that he decided to focus on midwifery, to which end he observed maternity care in Paris, among other cities.
In 1742, he married his cousin, Jane Wittingham, with whom he had two children:1 Charles and Jane, born in 1745 and 1746 respectively.