Alexis was born in Breslau, Silesia. His father, who came of a French refugee family, named Hareng, held a high position in the war department. He attended the Werdersche Gymnasium in Berlin, and then, serving as a volunteer in the campaign of 1815, took part in the siege of the Ardenne fortresses. On his return, he studied law at the University of Berlin and the University of Breslau and entered the legal profession, but he soon abandoned this career and devoted himself to literature. Settling in Berlin, he edited, from 1827 to 1835, the Berliner Konversationsblatt, in which for the first two years he was assisted by Friedrich Christoph Forster (1791-1868); and in 1828 was created a doctor of philosophy by the University of Halle. In 1852 he retired to Arnstadt in Thuringia, where after many years of broken health he died.
Having made his name first known as a writer by an idyll in hexameters, Die Treibjagd (1820), and several short stories, his literary reputation was first established by the historical romance Walladmor (1823), which was published as being "freely translated from the English of Sir Walter Scott, with a preface by Willibald Alexis". His novel Der Werwulf is set in Brandenburg at the time of the Protestant Reformation. In 1840 his historical novel Der Roland von Berlin was published. It was the basis for Ruggero Leoncavallo's opera of the same name.