Markstein also wrote for or story-edited other television series, specialising in espionage stories, and jointly ran a successful literary agency for screenwriters.

[4][5] Together with producer David Tomblin and the star McGoohan (uncredited), Markstein co-wrote the first story "Arrival," and then settled in as script editor for the series.

A glimpse of Markstein's face remained in the opening credits, but it was without him that McGoohan took the series to its most surreal and existential levels in the final four episodes, and to its bizarre conclusion.

In the same year, together with Jacqui Lyons, he co-founded the literary agency, Marjacq Scripts Ltd, initially to represent screenwriters, later also thriller writers and computer game authors.

In addition to his interests in Marjacq, Markstein also wrote several thrillers, including The Cooler (1974), The Man From Yesterday (1976), Chance Awakening (1977, basis of the screenplay Espion, lève-toi by Yves Boisset), the historical epic Tara Kane (1978), Goering Testament (1978), Traitor for a Cause (1979), Ultimate Issue (1981), Ferret (1983), and Soul Hunters (1987).

For the cinema he wrote the initial synopsis for the 1982 SAS embassy-storming film Who Dares Wins, which was then turned into a novel The Tiptoe Boys in thirty days flat by author James Follett and then into a screenplay by screenwriter Reginald Rose.