In addition to practicing after he was admitted to the bar, Badeau became a writer, and his early work as a theater critic was carried by Noah's Sunday Times.

Badeau joined the Union Army during the American Civil War, and his abilities as a writer led to his prominence as a staff officer, first for Thomas W. Sherman, and later for Ulysses S. Grant.

Badeau continued to work as a writer, and was a prolific contributor of essays and articles to newspapers and magazines, in addition to being the author of several books, both fiction and non-fiction.

[3] In addition, Badeau was a writer, and his work as an essayist and theater critic was published in Noah's Sunday Times.

Early in the next year he was made bearer of government dispatches to Madrid, then in May he was returned to London as Consul in 1870 and served in that capacity until September, 1881.

As Grant's Memoirs approached completion, having benefited from Badeau's extensive rewriting and additions to its earlier sections, he became convinced that sales would likely go far higher than the $30,000 envisioned in this agreement.

With Grant failing badly, Badeau proposed a new arrangement in April 1885: he would complete the work at the price of $1,000 a month, plus 10% of the profits.

Eventually Mrs. Grant paid him the originally agreed upon $10,000 plus interest of $1,200 (see Brooks D. Simpson article on Badeau in American National Biography).

[26] He was a prolific author of both fiction and non-fiction, and besides newspaper columns and magazine articles, his works included: The Vagabond (1859);[27] a three volume Military History of Ulysses S. Grant (1881);[28][12]  Aristocracy in England (1885);[29]  Conspiracy: A Cuban Romance (1885);[30] and Grant in Peace: From Appomattox to Mount McGregor (1887); It was subtitled A Personal Memoir, a work that covered Grant's political and social involvements with such notable people as General Philip Sheridan, Secretary of War Edwin M. Stanton, Secretary of State Hamilton Fish, and others.

In his final years Badeau continued writing, frequently contributing to various magazines and periodicals and chronicled the various war time events he participated in.

His frequent involvement as a writer caused strain on his eyes and he soon developed cataracts during the winter of 1894-1895 where he subsequently underwent successive operations for their removal, which undermined his physical strength.

[10] He finally succumbed to apoplexy, and died suddenly on March 19, 1895, in Ridgewood, New Jersey, at the age of 63[13][1][32] and was buried at the Old Dutch Church of Sleepy Hollow, Section D, Lot 65.