When the Spanish–American War broke out in 1898, Otis asked President William McKinley for an appointment as Assistant Secretary of War.
But Secretary of War Russell A. Alger did not want the conservative Otis serving under him.
Otis thereupon again volunteered for the Army and was appointed brigadier general of volunteers.
He served in the Philippines.
 He did not see any action against the Spanish, but commanded the 1st Brigade, 2nd Division, VIII Corps during the Philippine–American War.

His support for his adopted city was instrumental in the growth of the city.
He was a member of a group of investors who bought land in the San Fernando Valley based on inside knowledge that the Los Angeles Aqueduct would soon irrigate it.
In 1909, the Suburban Homes Company, a syndicate led by H. J. Whitley, general manager of the Board of Control, along with Harry Chandler, H. G. Otis, M. H. Sherman and O. F. Brandt purchased 48,000 acres of the Farming and Milling Company for $2,500,000.
Henry E. Huntington, extended his Pacific Electric Railway (Red Cars) through the Valley to Owensmouth (now Canoga Park).
 The Suburban Home Company laid out plans for roads and the towns of Van Nuys, Reseda (Marian) and Canoga Park (Owensmouth).
The rural areas were annexed into the city of Los Angeles in 1915.
On April 2, 1915 H. J. Whitley purchased the Suburban Home Company so that he would have complete control for finishing the development.

Otis and his son-in-law Harry Chandler and others formed the Colorado River Land Company, which bought land in the Mexicali Valley of Baja California at the turn of the twentieth century when Mexico's President Porfirio Díaz encouraged foreign investment to develop the country.
The company weathered the Mexican Revolution (1910-1920), but the rich agricultural land held in foreign hands was expropriated by the Mexican government during its postrevolutionary land reform.

On December 23, 1916, General Harrison Gray Otis, donated his spacious Wilshire Boulevard home across the street from MacArthur Park, known as the Bivouac, to Los Angeles County to be used “continuously and perpetually for the Arts and advancement of the Arts.”
The Otis Art Institute of the Los Angeles Museum of History, Science, and Art eventually became Otis College of Art and Design.
The home was torn down in the 1950s, but the school built new buildings and occupied the space until 1997.
It is now the site of a public elementary school.

He died on July 30, 1917 at the home of his son-in-law, Harry Chandler.
When the Spanish–American War broke out in 1898, Otis asked President William McKinley for an appointment as Assistant Secretary of War. But Secretary of War Russell A. Alger did not want the conservative Otis serving under him. Otis thereupon again volunteered for the Army and was appointed brigadier general of volunteers. He served in the Philippines.  He did not see any action against the Spanish, but commanded the 1st Brigade, 2nd Division, VIII Corps during the Philippine–American War.
His support for his adopted city was instrumental in the growth of the city. He was a member of a group of investors who bought land in the San Fernando Valley based on inside knowledge that the Los Angeles Aqueduct would soon irrigate it. In 1909, the Suburban Homes Company, a syndicate led by H. J. Whitley, general manager of the Board of Control, along with Harry Chandler, H. G. Otis, M. H. Sherman and O. F. Brandt purchased 48,000 acres of the Farming and Milling Company for $2,500,000. Henry E. Huntington, extended his Pacific Electric Railway (Red Cars) through the Valley to Owensmouth (now Canoga Park).  The Suburban Home Company laid out plans for roads and the towns of Van Nuys, Reseda (Marian) and Canoga Park (Owensmouth). The rural areas were annexed into the city of Los Angeles in 1915. On April 2, 1915 H. J. Whitley purchased the Suburban Home Company so that he would have complete control for finishing the development.
Otis and his son-in-law Harry Chandler and others formed the Colorado River Land Company, which bought land in the Mexicali Valley of Baja California at the turn of the twentieth century when Mexico's President Porfirio Díaz encouraged foreign investment to develop the country. The company weathered the Mexican Revolution (1910-1920), but the rich agricultural land held in foreign hands was expropriated by the Mexican government during its postrevolutionary land reform.
On December 23, 1916, General Harrison Gray Otis, donated his spacious Wilshire Boulevard home across the street from MacArthur Park, known as the Bivouac, to Los Angeles County to be used “continuously and perpetually for the Arts and advancement of the Arts.” The Otis Art Institute of the Los Angeles Museum of History, Science, and Art eventually became Otis College of Art and Design. The home was torn down in the 1950s, but the school built new buildings and occupied the space until 1997. It is now the site of a public elementary school.
He died on July 30, 1917 at the home of his son-in-law, Harry Chandler.
Harrison Gray Otis (publisher)