That he was a member of the gentry is evidenced by the coat of arms stamped on his pewter plate and he appears to have been a local Commissioner of the Peace.
He found some recreation in questioning the vagrants who begged at his door as to their modes of life and paid frequent visits to London with the object of corroborating his information.
Occasionally, his indignation was so roused by the deception practised by those whom he interrogated at his own door that he took their licenses from them and confiscated their money, distributing it among the honest poor of his neighbourhood.
Its popularity was at once so great that Henry Bynneman and Gerrard Dewes were both fined by the Stationers' Company in 1567, for attempting to circulate pirated copies.
A dedication by Harman to his neighbour, Elizabeth, Countess of Shrewsbury, widow of the 4th Earl, who held the manor of Erith and "the epistle to the reader" is followed by exhaustive small essays on 24 classes of the thieves' and tramps' fraternity, and by a list of names of the chief professors of the art "lyuinge nowe at this present".
The Groundworke of conny-catching (1592), very doubtfully assigned to Robert Greene, reprints the greater part of Harman's book.
Dekker, in the second part of his Belman, called Lanthorne and Candlelight (1609), conveyed to his pages Harman's vocabulary of thieves' words, which Richard Head incorporated in his "English Rogue" (1671–80).