At was while he was in college that Ridpath began publishing essays, sketches, and poems. He switched to books of history and, in 1874, published his important Academic History of the United States. Similar books followed. In 1880, he retired from his educational career and focused on writing, publishing biographies of President James Garfield and others.
His major work, however, was the 16-volume Ridpath's History of the World (or Cyclopedia of Universal History), first published in 1894 and illustrated with over 3000 images. His goal was to present a history of the entire human race "from the beginnings of civilization to the present time." Several editions were published in the next several decades. In his preface, Ridpath explains that "mankind is not an event, but a producing force" and, as such, will not write a book like other traditional histories. Further, he says, human civilization is like a river which shapes the landscape and moves resources from one place to another. Though other books have focused on the works, his subject was what created those works in general; "What of the river itself?" he asks:
It pours along, gathering and increasing. It is an entity. It may well seem alive. It moves and roars and rushes. Its volume is measurable, but becomes immeasurable with increase. Its color is of this tint. Its water has this quality or that. Its manner is placid, smiling, gentle, or angry, turbulent, stormy. It sleeps or wakes. It rejoices with sunshine or calm, moans with the pressure of shadow and tempest, becomes furious, and springs with madness through narrowing gorges and over horrid precipices. It yields to the rigor of winter, and bursts with the renewal of spring. All this is the river itself. All this has respect to the substance and life of the great fact, and not to its results and reactions.
The analogue of the river is the human race. That, too, is a stream flowing from an invisible fountain. That also has had its sources in the highlands of the past, and that also has gathered and rolled down with increasing volume into the plains of the present. Like the river, the human race possesses a life of its own. It is an entity dividing into many entities. It spreads far and covers the earth with its floods. It leaves on all shores and continents the signs of its presence and activities. It builds up and demolishes. It changes its course according to the exigencies of the physical barriers that are set against its progress. It breaks through and traverses vast regions. It modifies the whole globe, and determines both its material and its immaterial aspects. It becomes the one really important fact on the earth.