The film is a heavily fictionalised account of the murderous witch-hunting exploits of Matthew Hopkins (Price), a lawyer who falsely claimed to have been appointed as a "Witch Finder Generall" [sic] by Parliament during the English Civil War to root out sorcery and witchcraft.

Several prominent critics have championed the film, including Tim Lucas, J. Hoberman, Danny Peary, Robin Wood and Derek Malcolm; their praise has highlighted its direction, performances, and musical score by Paul Ferris.

In 1645, during the English Civil War, Matthew Hopkins, an opportunistic witch hunter, takes advantage of the breakdown in social order to impose a reign of terror in East Anglia.

After surviving a brief skirmish and killing his first enemy soldier (and thus saving the life of his Captain), he rides home to Brandeston, Suffolk, to visit his lover Sara, the niece of John Lowes, the village priest.

He has needles stuck into his back (in an attempt to locate the so-called "Devil's Mark") and is about to be killed when Sara stops Hopkins by offering him sexual favours in exchange for her uncle's safety.

[6] Michael Reeves, who had just completed Tigon's The Sorcerers (1967) starring Boris Karloff, provided a story outline which met with Tenser's enthusiastic approval.

[7][8] With the abrupt change of star, Reeves and Baker had to rethink their original concept of presenting Hopkins as "ineffective and inadequate ... a ridiculous authority figure", which they had believed Pleasence could perform effectively.

They knew the tall, imposing Price, with his long history of horror roles, would have to be more of a straightforward villain, and they made changes to their script accordingly.

On the same day, a preliminary report was issued by a BBFC examiner, who, commenting that Tenser was an "ape", referred to the screenplay as "perfectly beastly" and "ghoulish".

In the original ending, Stearne falls in with a group of gypsies and attempts to rape one of their women, who successfully fights off her attacker by plunging her thumbs into his eyes, blinding him.

Tenser had previously expressed concerns regarding the scope of the Battle of Naseby sequence as well as the gypsy-ending, as these scenes would both require the employment of additional groups of extras.

Describing his working relationship with Reeves, Ogilvy observed that "his mastery of the technical aspects was absolute", but added "Mike never directed the actors.

[17] Appearing as Dwyer's uncle, Witchfinder was one of several horror films the British character actor Rupert Davies performed in during the later stage of his career.

AIP heads Samuel Z. Arkoff and James H. Nicholson did not expect a high quality result; the movie was intended to be a tax write-off.

When the scene was filmed, Ogilvy responded with blows that were not faked, but Waddilove had fitted Price's costume with padding, protecting the actor from injury.

[38] Reeves refused to take part in the filming of these sequences and they were completed by the crew after the initial versions of the scenes had been shot, with Tigon's Tenser acting as director.

He drew inspiration from the folk song "Greensleeves" in writing the romantic theme "Peaceful Interlude" as a means of evoking its time period, as well as to serve as a counterpoint to the film's violence.

[42] All music is composed by Paul FerrisIn a piece written for the British Film Institute commemorating the 50th anniversary of Witchfinder General's original release, Adam Scovell identified "weaponised belief"—represented by Hopkins's exploitation of the irrationalities and superstitions of the populace as a means of gaining power and fulfilling his sexual and political ambitions—as the film's primary thematic concern, stating that "belief doesn't create [his] sword, but it most definitely sharpens it".

[2] Writing for Cine Outsider, Jerry Whyte believes that the film "brilliantly recreates that sense of social collapse" and its commentary has merit in critiquing the policies of historical witch-hunts, which would have resonated with contemporary audiences in the light of McCarthyism and the Vietnam War.

[44] Expanding on Iain Sinclair's assertion that, "The film's success lies in the tension between [Tom] Baker's Utopian permissiveness, his feel for the country, and Reeves' demonic fatalism",[45] Whyte describes Witchfinder as an "English Western".

[44] Noting Reeves' long-standing interest in the genre, he identifies the film's various connections to it, including its frequent horse-riding sequences, clear distinctions between good and evil, and Marshall's pursuit of revenge against Hopkins.

[44] Witchfinder General is commonly cited as an example of the subgenre of folk horror and is often grouped with The Blood on Satan's Claw (1971) and The Wicker Man (1973) as definitive works of the genre.

[50] Scovell connects these films to folk horror because in all three, characters are physically and/or socially isolated in a rural landscape where they confront hostile belief systems.

"[54] Reeves agreed to make some of the initial minor cuts himself, but when additional and more extensive demands were made he adamantly refused to take part in any further editing.

In The Sunday Times, Dilys Powell emphasised the "hanging, burning, raping, [and] screaming" and called the film "Peculiarly nauseating.

[58] AIP heads Arkoff and Nicholson originally contributed their portion of the budget as a tax write-off, but when they were screened the completed film they were astonished by its quality.

"[13] In the US, the film was not subject to any censorship and released to AIP's usual mix of drive-ins and grindhouses on a double bill with The Young, the Evil and the Savage.

[85][86] Witchfinder General was the inspiration for a BBC Radio 4 play Vincent Price and The Horror of The English Blood Beast, first broadcast in March 2010.

[92] Tim Lucas described Schmidt's rescoring as a "betrayal to every effort the original film made to remain true to its time frame".

[97] Scream Factory included the film as part of the company's multi-title Vincent Price Blu-ray box set released in fall of 2013.