Amongst his other activities, Henry Parker was a political writer. He was celebrated in his own day (though anonymously) as the Observator, theauthor of Observations upon Some of His Majesties Late Answers and Expresses of June 1642, a widely read and attacked reply to recent royalist declarations. Parker's reputation today is much the same. To notice just a few recent and more or less interchangeable remarks, Parker was “the most formidable parliamentary apologist of the First Civil War,” “the most important of parliament's proponents,” and “the most original and daring of all Parliament's pamphleteers in grasping the fundamental issue of sovereignty.” Observations was “perhaps the most influential pamphlet of the entire civil war”.

Then and now, pamphleteering gained Parker much the greater part of the regard he had in the world. Yet for Parker's own day this is necessarily asardonic comment. Much of what Parker wrote appeared anonymously, and not many people seem to have been well informed of the particulars. Apparently none of the many critics of Observations knew the baptismal name of the Observator. And although there was at least one published revelation of the Observator's identity in 1649, Parker (like John Locke with respect to the Two Treatises) never admitted his authorship.Indeed the fullness of Parker's contribution to political thought might not have come to light, were it not for the accidents of Parker's friendship with the publisher and collector George Thomason, and, to a lesser extent, his Oxford degrees, which made him a target of the antiquarian labors of Anthony a Wood. Had his friendships or his university been different, Parker might be remembered primarily for a different sort of achievement.