The son of George Todhunter, a Nonconformist minister, and Mary née Hume, he was born at Rye, Sussex. He was educated at Hastings, where his mother had opened a school after the death of his father in 1826. He was at first at a school run by Robert Carr, moving then to one opened by John Baptist Austin.^[1]

Todhunter became an assistant master at a school at Peckham, attending at the same time evening classes at the University College, London where he was influenced by Augustus De Morgan. In 1842 he obtained a mathematical scholarship and graduated as B.A. at London University, where he was awarded the gold medal on the M.A. examination. About this time he became mathematical master at a school at Wimbledon.^[2]

In 1844 Todhunter entered St John's College, Cambridge, where he was senior wrangler in 1848, and gained the first Smith's Prize and the Burney Prize; and in 1849 he was elected to a fellowship, and began his life of college lecturer and private tutor.^[3] In 1862 he was made a fellow of the Royal Society,^[4] and in 1865 a member of the Mathematical Society of London. In 1871 he gained the Adams Prize and was elected to the council of the Royal Society. He was elected honorary fellow of St John's in 1874, having resigned his fellowship on his marriage in 1864. In 1880 his eyesight began to fail, and shortly afterwards he was attacked with paralysis.^[2]

He is buried in the Mill Road cemetery, Cambridge.^[2]

Todhunter married 13 August 1864 Louisa Anna Maria, eldest daughter of Captain (afterwards Admiral) George Davies, R.N. (at that time head of the county constabulary force). He died on 1 March 1884, at his residence, 6 Brookside, Cambridge. A mural tablet and medallion portrait were placed in the ante-chapel of his college by his widow, who, with four sons and one daughter, survived him.^[2]

He was a sound Latin and Greek scholar, familiar with French, German, Spanish, Italian, and also Russian, Hebrew, and Sanskrit. He was well versed in the history of philosophy, and on three occasions acted as examiner for the moral sciences tripos.^[2]

• Treatise on the Differential Calculus and the Elements of the Integral Calculus (1852, 6th ed., 1873)
• Treatise on Analytical Statics (1853, 4th ed., 1874)
• Treatise on the Integral Calculus (1857, 4th ed., 1874)
• Treatise on Algebra (1858, 6th ed., 1871)
• Treatise on differential Calculus
• Treatise on Plane Coordinate Geometry (1858, 3rd ed., 1861)
• Plane Trigonometry (1859, 4th ed., 1869)
• Spherical Trigonometry (1859)
• History of the Calculus of Variations (1861)
• Theory of Equations (1861, 2nd ed. 1875)
• Examples of Analytical Geometry of Three Dimensions (1858, 3rd ed., 1873)
• Mechanics for Beginners (1867)
• A History of the Mathematical Theory of Probability from the Time of Pascal to that of Laplace (1865)
• Researches in the Calculus of Variations (1871)
• History of the Mathematical Theories of Attraction and Figure of the Earth from Newton to Laplace (1873)
• Elementary Treatise on Laplace's, Lamé's and Bessel's Functions (1875)
• A history of the theory of elasticity and of the strength of materials from Galilei to the present time Vol I PtI Vol II Pt II
• Natural Philosophy for Beginners (1877).

An unfinished work, The History of the Theory of Elasticity, was edited and published posthumously in 1886 by Karl Pearson. A biographical work on William Whewell was published in 1876, in addition to many original papers in scientific journals.

Todhunter wrote textbooks on algebra and trigonometry, and a revision of the translation by Robert Simson of Euclid's Elements, which, with an introduction by Thomas Little Heath, was republished by Everyman in 1933.^[5]

Todhunter's major historical works include a history of the Probability theory from the time of Blaise Pascal to that of Pierre-Simon Laplace first published in 1865.^[6][7]

Some of these are available at Isaac Todhunter's publications at Google Books.

Attribution:   This article incorporates text from a publication now in the public domain: Lee, Sidney, ed. (1898). "Todhunter, Isaac". Dictionary of National Biography. Vol. 56. London: Smith, Elder & Co.

• Obituary notices: Proc. Lond. Math. Soc. (1884) and Proc. Roy. Soc. 37, p. xxvvii (1884)

A digital version of the above obituary is at the Gallica site.

• Obituary, The Eagle: A Magazine Support by Members of St. John's College. W. Metcalfe. 1885. pp. 94–98.
• Chisholm, Hugh, ed. (1911). "Todhunter, Isaac" . Encyclopædia Britannica. Vol. 26 (11th ed.). Cambridge University Press. p. 1044.
• Mullinger, J. B.; Rice, Adrian. "Todhunter, Isaac (1820–1884)". Oxford Dictionary of National Biography (online ed.). Oxford University Press. doi:10.1093/ref:odnb/27493. (Subscription or UK public library membership required.)

• O'Connor, John J.; Robertson, Edmund F., "Isaac Todhunter", MacTutor History of Mathematics Archive, University of St Andrews
• Isaac Todhunter (1873). The conflict of studies, and other essays on subjects connected with education. Macmilla and Company. p. 33.
• Isaac Todhunter (1876). William Whewell, D. D., Master of Trinity College, Cambridge: An Account of His Writings with Selections from His Literary and Scientific Correspondence. Macmillan and Company. ISBN 9780598006356.
• Works by Isaac Todhunter at Project Gutenberg
• Works by or about Isaac Todhunter at Internet Archive