After graduating, he spent a year in charge of the Seminary in Chester, Geauga County, Ohio.
In 1862, during the American Civil War, Vermont governor Holbrook had him appointed Paymaster with the rank of major.
He spent the next three years in various stations in Virginia and the Department of the Gulf, Washington, Cincinnati, and other Northern posts.
He and the other paymasters were prepared to burn the money in their custody if Longstreet took the city, even though the general sent word he would hang them if they did this.
[2] After the War, Walker moved to New York City and joined the law office of Charles H. Hunt, the assistant United States District Attorney under Dickinson.
He resigned from the Board in 1886, when he was appointed the United States Attorney for the Southern District from New York.
He joined the New York City Bar Association in 1870, serving as vice-president from 1889 to 1891 and as a member of the Executive Committee at the time of his death.