He used to be a government lawyer.

In 1901, during the second term of the McKinley presidency, Mosby moved back to Washington and started looking for work in the Department of Justice once more. President Theodore Roosevelt dispatched Mosby to the west as an interior department representative following McKinley's killing. There, Mosby dealt with illegal fencing by cattle barons in Nebraska and Colorado who frequently made up stories about the lodging of war widows and broke the law. erred by breaking the 1885 Van Wyck Fence Act.

Due to witnesses' concerns about retaliation, Mosby sent the first notification required by law to the impacted landowners after they declined to testify about the illegal fencing. Mosby relocates to western Nebraska after learning that the land agent actually resides in Iowa and does not manage the range because the Pawnee Cattle Company has its headquarters there.

Mosby was ordered to intercept trespassers in Alabama woodland after being called back to Washington to placate Nebraska senators. But additional attorneys were sent, and they succeeded in getting the charges. When Philander Knox's tenure as attorney general came to an end, Mosby finally received the Justice Department position he desired. However, Mosby served for his brother-in-law Charles W. Russell, Jr. at the Bureau of Territorial and Territorial Affairs, not the trustee, from 1904 until 1910. Roosevelt was sent back to Alabama in 1905 to refute claims that there were anomalies in Mobile Harbor. He was subsequently dispatched to Oklahoma to look into accusations made against US Marshal Benjamin Colbert.