Clement Mansfield Ingleby (29 October 1823 – 26 September 1886) was an English Shakespearian scholar, perhaps best remembered as John Payne Collier's nemesis.
Clement Ingleby was born at Edgbaston near Birmingham, the son of a well-known lawyer. Poor health – he was not expected to live long – kept him from attending school, so he was privately educated at home. He went to Trinity College, Cambridge, when twenty years old, and specialised in mathematics. He received his B.A. in 1847 and his M.A. in 1850. Returning to Birmingham he went to work in his father's law office, and then became a partner in the firm of Ingleby, Wragge, and Ingleby (now Wragge & Co LLP). In spite of his poor health he devoted his spare time to metaphysics, mathematics, and English literature.
In 1850 Ingleby married Sarah Oakes (d. 3 January 1906).
In 1855 the Birmingham and Midland Institute was established, an experiment in continuing adult education. Ingleby took on giving a class in logic and metaphysics at the industrial branch. His methods were novel and the class was successful. A disciple of William Hamilton, Ingleby focused on the most current views, even obtaining from Hamilton his yet-unpublished improvements. Urged by his students, Ingleby issued Outlines of Theoretical Logic in 1856 as a textbook in the subject. It was his first published volume.
In the 1850s documents discovered by John Payne Collier bearing on Elizabethan stage history in general and Shakespeare's life in particular fell under suspicion. Re-examination of several documents showed them to be out-and-out forgeries, forgeries so obvious it was difficult to see how Collier could have been deceived by them. One item of particular interest, the Perkins Folio, had never been examined by anybody besides Collier. It contained many corrections in what appeared to be a 16th-century hand that Collier suggested might be based on stage tradition. Ingleby, along with Sir Frederick Madden, who put the resources of the British Museum on the task, were finally able to examine the Perkins Folio in detail. They discovered—as was the case with others of the forgeries—modern pencil-marks under the supposedly ancient writing. The handwriting of these appeared to be Collier's.