Allen was forced to end his studies upon his father's death. While he volunteered for militia service in 1757 in response to French movements resulting in the siege of Fort William Henry, his unit received word while en route that the fort had fallen, and turned back. Even though the French and Indian War continued over the next several years, Allen did not apparently participate in any further military activities, and is presumed to have tended his farm, at least until 1762. In that year, he became part owner of an iron furnace in Salisbury. He also married Mary Brownson, a woman five years his senior, from the nearby town of Roxbury, in July 1762. They first settled in Cornwall, but moved the following year to Salisbury with their infant daughter Loraine. Allen bought a small farm and proceeded to develop the iron works. The expansion of the iron works was apparently costly to Allen; he was forced to sell off portions of the Cornwall property to raise funds, and eventually sold half of his interest in the works to his brother Heman. The Allen brothers sold their interest in the iron works in October 1765.