While at a Scabbard and Blade honors society conference in New Orleans, Louisiana, Shoup heard a speech by Major General John A. Lejeune, the Commandant of the Marine Corps, offering commissions in the U.S. Marine Corps to interested officer candidates.
Shortly after being commissioned as a second lieutenant in the U.S. Army Reserve in May 1926, Shoup applied, and was offered a commission in the Marine Corps.
In August 1926 he resigned his commission in the army and traveled from Camp Knox, Kentucky, to Chicago, Illinois, for physical exams.
On 25 August 1926, he arrived at the Marine Corps barracks at the Philadelphia Navy Yard, where he was commissioned as a second lieutenant, and commenced Marine Officers Basic School.
Shoup maintained that he had never previously considered a military career and had only chosen to become a military officer for the pay.
Throughout his service, he excelled in athletics and marksmanship, and during his early career he would coach recreational athletic teams in addition to his regular assignments.
He quickly established himself as an assertive and demanding leader, impressing both his commanders and his subordinates.
Despite his no-nonsense demeanor, those he commanded later recalled his ability to keep morale with his sense of humor.
He often sported a clenched cigar, which became something of a trademark during his front-line service.

On 1 April 1927, Shoup and nine other officers were pulled from training to accompany a detachment of Marines sent to China to protect American interests during the instability of the Chinese Civil War.
Embarking from San Diego with the 2nd Battalion, 10th Marines, Shoup's contingent was to play a primarily defensive role, and not engage in military action.
Still, he was personally opposed to the mission, feeling that Americans in China were exploiting its people.
The contingent landed in Shanghai on 10 June aboard USS Chaumont.
Initially, they conducted shore patrol around American sections of the city.
On 5 July the battalion was moved to Tientsin to protect American interests there should Chinese Nationalist troops threaten them.
Shoup became seriously ill there, and remained hospitalized until it was announced U.S. troops would leave.
He briefly returned to Shanghai to oversee foreign troop departures with the 4th Marines, before he departed from China on 7 December 1928.

Following this assignment, Shoup returned to the United States and completed his training.
He then spent short stints at Marine bases in Quantico, Virginia, Pensacola, Florida, and San Francisco, California.
From June 1929 to September 1931 he served with the Marine Detachment aboard USS Maryland, where he coached the boxing and wrestling squads.
Following this duty he was assigned to the Marine Corps Recruit Depot San Diego.
In May 1932 he was ordered to Puget Sound Navy Yard in Bremerton, Washington, where he was promoted to first lieutenant one month later.
He served on temporary duty with the Civilian Conservation Corps in Idaho and New Jersey from June 1933 to May 1934, after which he returned to Bremerton.

Shoup went back to China in November 1934, again serving briefly with the 4th Marines in Shanghai.
He was soon reassigned as a legation guard in Peiping, where he taught the post's pistol and rifle teams to shoot competitively.
They won at least one major competition.
He also had time to observe the troops of the Empire of Japan, gaining great respect for their discipline.
In 1936, he came down with a serious case of pneumonia and had to be evacuated from China.
His next duty was at Puget Sound Navy Yard.
In October 1936, he was promoted to captain.
In July 1937 he entered Junior Course, Marine Corps Schools in Quantico, which he completed in May 1938.
He then served as an instructor and Plans & Training officer with Reserve Officers Class at Quantico for two years.
In June 1940 he joined the 6th Marines in San Diego, and was promoted to major in April 1941.
While at a Scabbard and Blade honors society conference in New Orleans, Louisiana, Shoup heard a speech by Major General John A. Lejeune, the Commandant of the Marine Corps, offering commissions in the U.S. Marine Corps to interested officer candidates. Shortly after being commissioned as a second lieutenant in the U.S. Army Reserve in May 1926, Shoup applied, and was offered a commission in the Marine Corps. In August 1926 he resigned his commission in the army and traveled from Camp Knox, Kentucky, to Chicago, Illinois, for physical exams. On 25 August 1926, he arrived at the Marine Corps barracks at the Philadelphia Navy Yard, where he was commissioned as a second lieutenant, and commenced Marine Officers Basic School. Shoup maintained that he had never previously considered a military career and had only chosen to become a military officer for the pay. Throughout his service, he excelled in athletics and marksmanship, and during his early career he would coach recreational athletic teams in addition to his regular assignments. He quickly established himself as an assertive and demanding leader, impressing both his commanders and his subordinates. Despite his no-nonsense demeanor, those he commanded later recalled his ability to keep morale with his sense of humor. He often sported a clenched cigar, which became something of a trademark during his front-line service.
On 1 April 1927, Shoup and nine other officers were pulled from training to accompany a detachment of Marines sent to China to protect American interests during the instability of the Chinese Civil War. Embarking from San Diego with the 2nd Battalion, 10th Marines, Shoup's contingent was to play a primarily defensive role, and not engage in military action. Still, he was personally opposed to the mission, feeling that Americans in China were exploiting its people. The contingent landed in Shanghai on 10 June aboard USS Chaumont. Initially, they conducted shore patrol around American sections of the city. On 5 July the battalion was moved to Tientsin to protect American interests there should Chinese Nationalist troops threaten them. Shoup became seriously ill there, and remained hospitalized until it was announced U.S. troops would leave. He briefly returned to Shanghai to oversee foreign troop departures with the 4th Marines, before he departed from China on 7 December 1928.
Following this assignment, Shoup returned to the United States and completed his training. He then spent short stints at Marine bases in Quantico, Virginia, Pensacola, Florida, and San Francisco, California. From June 1929 to September 1931 he served with the Marine Detachment aboard USS Maryland, where he coached the boxing and wrestling squads. Following this duty he was assigned to the Marine Corps Recruit Depot San Diego. In May 1932 he was ordered to Puget Sound Navy Yard in Bremerton, Washington, where he was promoted to first lieutenant one month later. He served on temporary duty with the Civilian Conservation Corps in Idaho and New Jersey from June 1933 to May 1934, after which he returned to Bremerton.
Shoup went back to China in November 1934, again serving briefly with the 4th Marines in Shanghai. He was soon reassigned as a legation guard in Peiping, where he taught the post's pistol and rifle teams to shoot competitively. They won at least one major competition. He also had time to observe the troops of the Empire of Japan, gaining great respect for their discipline. In 1936, he came down with a serious case of pneumonia and had to be evacuated from China. His next duty was at Puget Sound Navy Yard. In October 1936, he was promoted to captain. In July 1937 he entered Junior Course, Marine Corps Schools in Quantico, which he completed in May 1938. He then served as an instructor and Plans & Training officer with Reserve Officers Class at Quantico for two years. In June 1940 he joined the 6th Marines in San Diego, and was promoted to major in April 1941.
David M. Shoup