On his first day on the job, the new Interior Department secretary, Ryan Zinke, proved he knows how to make an entrance: arriving high in the saddle on Tonto, an Irish sport horse. Mounted police escorted him a few blocks through Washington.

Mr. Zinke, a fifth-generation Montanan who now oversees the country’s 500 million acres of public land, including 59 national parks, showed up to work to a grand reception.

Officers from various agencies under the Interior Department lined the steps to the administration building. A drummer from his home state’s Northern Cheyenne tribe performed. The department is the liaison with federally recognized American Indian and Alaska Native nations.

Mr. Zinke, 55, served nearly two dozen years with the Navy SEALs before entering politics in 2008 as a Republican. After two terms in the Montana legislature, he was elected in 2014 as the state’s at-large representative in the House, before his nomination to the cabinet by President Trump. He was confirmed on Wednesday by the Senate with a vote of 68-31.