Alsop was baptized at St. Martin in the Fields in June 1636.
Despite his wide reading, evident in his book, A Character of the Province of
Mary-Land (1666), Alsop was probably born into humble circumstances. A
loyal follower of King Charles I, Alsop seems to have feared unhappy political
recriminations when Oliver Cromwell rose to power, so he signed an indenture
that brought him to Maryland in 1658. He spent four years of his indenture near
Baltimore before returning to England, where he wrote A Character, a
lively promotional tract that blended pastoral and commercial themes.