Kermit was born at the Roosevelt residence Sagamore Hill in Oyster Bay, New York, the second child born to Theodore Roosevelt and his second wife, Edith Kermit Carow Roosevelt. Kermit's oldest brother was Theodore Jr. and his younger siblings were Ethel, Archibald, and Quentin. His older half sister was Alice, from his father's first marriage to Alice Roosevelt.

As a child, Kermit Roosevelt had little resistance to illness and infection. He had a flair for language, however, and was an avid reader. He showed a talent for writing that led to recording his experiences in World War I in a book.

After attending the Groton School, Kermit attended college at Harvard. In 1909 as a freshman, he and his father (recently out of office as President), both of whom loved nature and outdoor sports went on a safari in Africa. After this trip and a swing through Europe, Kermit returned to Harvard and completed four years of study in two and one-half years. He was a member of the Porcellian Club.

One of Theodore Roosevelt's most popular books, Through the Brazilian Wilderness, recounted the father-and-son expedition into the Amazon Basin Brazilian jungle in 1913—14. Kermit and TR went on what would become known as the Roosevelt-Rondon Scientific Expedition, exploring the Brazilian jungle with Brazilian explorer Colonel Cândido Rondon. During this expedition, they explored the Rio of Doubt, later renamed Rio Roosevelt in honor of the President, as well as a branch of that river named the Rio Kermit in Kermit's honor. The source of the river had been discovered by Rondon earlier, but it had never been fully explored or mapped.

At the time of the expedition, Kermit was newly engaged to Belle Willard, daughter of the U.S. ambassador to Spain. Kermit's mother, Edith, was concerned about her husband's health and the difficulties of a new expedition and asked Kermit to accompany his father. Kermit did not want to delay his marriage, but decided it best to go with his father, and it was fortunate that he did so.

A far less ambitious expedition had originally been intended, and the early planning had been made with this more leisurely trip in mind. As a result, the participants were not entirely prepared for the true journey scope: An exploratory trip made to trace the River of Doubt from its source, to determine its path through hundreds of kilometers of uncharted rainforest. The difficulties of the harsh climate with its torrential downpours, rough terrain, shoddy low-running dugout canoes, a seemingly endless series of difficult rapids and waterfalls, diminishing food supplies, the drowning of one expedition member and the murder of another, and a host of other problems turned what began as a scientific expedition into a race against time to save the life of a dying former president. Malaria and a serious infection resulting from a minor leg wound had developed into a life-threatening situation. These illnesses so weakened TR, that by six weeks into the expedition, TR had to be attended day and night by the expedition's physician and his son, Kermit. TR considered his own condition a threat to the survival of the others. With the loss of many of their dugout canoes and provisions, and his father sick from malaria and infections, Kermit raced against time to bring his father back alive from the jungle. In a letter to a friend later, TR recalled considering taking a lethal dose of morphine because his sickness had reduced him to total dependence on the others and this wasn't the Roosevelt way. Kermit courageously stood up to the dying old man, and told him that he was bringing him back literally "dead or alive" and if he died, he would be an even bigger burden to the expedition. Without a doubt, it was Kermit who saved his father's life when this expedition had degenerated into a horrible ordeal.

During that expedition, Kermit himself came close to death by malaria. In order to save quinine for his father, he downplayed his own malarial sickness until the expedition's doctor was forced to give it to him by injection. By that time, his attempts to disguise his losing fight with the disease had come close to killing him.

TR was having chest pains when he tried to walk, his temperature soared to 103 and, at times he was delirious. By now so weakened that he could not even sit up in his dugout, he had to lie on his back. When the expedition finally reached civilization, TR had to be carried off by stretcher. He had lost over fifty pounds. Kermit and all the expedition's members' physical conditions had suffered as well.

In the final analysis, without Kermit's rope and canoe-handling skills, which preserved the dugouts from destruction (the one thing that would have quickly and fatally ended the expedition), his unflinching courage, dogged determination, - in short, the devotion and loving support of a dedicated son, it is unlikely that TR would have survived the expedition. Besides the newly-named Rio Roosevelt, one branch of the river was named the Rio Kermit in Kermit's honor. Today, the Rio Roosevelt is commonly called the Rio Teodoro by Portuguese-speaking Brazilians because of pronunciation difficulties they have with the name 'Roosevelt'.

Upon his return by ship to New York, friends and family were startled at TR's physical appearance, for he was no longer the vibrant man with a seemingly endless supply of energy that they had always known. Indeed, TR would write a friend that the trip had cut his life short by ten years. He did not realize, at that time, just how accurate his analysis would prove to be. The effects of the South America expedition had so greatly weakened TR that they significantly contributed to his declining health. For the rest of his life, he would be plagued by flareups of malaria and inflammation so severe that they would require hospitalization.

When TR and Kermit had recovered from their ordeal, they found that they had a new battle on their hands. In professional circles, there was doubt about their claims of having discovered and navigated a completely uncharted river over 1000 km long. TR and Kermit had to fight for official recognition of the expedition's discovery of the newly-named Rio Roosevelt. Toward this end, TR went to Washington, D.C., and spoke at a standing-room-only convention to defend his discovery. His official report and its defense silenced the critics and TR was able to triumphantly return to his home in Oyster Bay.

TR would record these harrowing experiences in one of his most popular books, Through the Brazilian Wilderness. The 1913—14 expedition would later be recounted in The River of Doubt by Candice Millard (Doubleday 2005).

After the Amazon trip, in 1914 Kermit married Belle Wyatt Willard (1892—1968), daughter of the U.S. Ambassador to Spain. They had four children:

• Kermit "Kim" Roosevelt (1916—2000) Kermit Roosevelt, Jr. was the mastermind of the Central Intelligence Agency's (CIA) Operation Ajax, which orchestrated the coup against Iran's democratically-elected Mohammed Mossadegh administration, and returned Mohammad Reza Pahlavi, the Shah of Iran, to Iran's Peacock Throne in August 1953 for the purpose of returning Western control of Middle Eastern oil supplies.
• Joseph Willard Roosevelt (1918—2008)
• Belle "Clochette" Wyatt Roosevelt (1919—1985)
• Dirck Roosevelt (1925—1953)

From 1914 to 1916, Kermit was assistant manager for National City Bank in Buenos Aires.

In 1917 as he was about to be transferred to a Russian branch, the U.S. entered the World War. On 22 August 1917, Kermit was appointed an honorary captain in the British Army, and saw hard fighting in the Near East, later transferring to the United States Army. While his other brothers had had summer training at Plattsburg, New York, Kermit had missed out on this training.

Kermit joined the British Army to fight in the Mesopotamia (modern-day Iraq) theater of World War I. He was attached to the 14th Light Armoured Motor Battery of the Machine Gun Corps, but the British High Command decided they could not risk his life and so they made him an officer in charge of transport (Ford Model T cars). From then on, however, Captain Roosevelt made it his main aim in life to get his Ford in front of the armor. With his incredible talent for languages, within months of being posted to Mesopotamia, he had mastered spoken as well as written Arabic and was often relied upon as a translator with the locals. As in Africa with his father, he was courageous to the point of recklessness. He was awarded a Military Cross on 26 August 1918. When the United States joined the war, Kermit got transferred to the AEF in Europe, relinquishing his British commission on 28 April 1918. In 1918, he learned that his youngest brother, Quentin, a pilot, had been shot down over France and had been buried by the Germans with full military honors.

After the expedition, Roosevelt went into business; he founded the Roosevelt Steamship Company and the United States Lines. Ever a Roosevelt, Kermit continued to enjoy outdoor activities with his brothers.

In 1929, Kermit along with his brother Theodore Roosevelt II went on an epic hunting expeditions across the Himalayas over uncharted mountain passes rising from the famous Vale of Kashmir through the ancient Silk Route into China in search of the legendary big horn wild sheep called "Ovis Poli", it is conceded by sportsmen the world over to be one of the finest of all game trophies. Kermit has documented the travel through his exceptional writing ability in his book titled "East of the Sun and West of the Moon". Several trophies collected during this expedition are on display in the Field Museum of Natural History in Chicago.

By 14 October 1939, when Britain was at war with Germany, Kermit had negotiated a commission as a Second Lieutenant in the Middlesex Regiment with the assistance of his friend, Winston Churchill, who was by then First Lord of the Admiralty (Churchill would later become Prime Minister after Chamberlain's downfall, spurred by the sudden German assault in France in May 1940). His first task was to lead a contingent of British volunteers for the Winter War in Finland. According to a contemporary story published in Picture Post, he had resigned from the British Army to lead the expedition. This story was probably a necessary cover so that he would be able to travel with the volunteers through neutral countries. However, before the expedition could be launched, Finland was forced to make peace with Russia. Kermit served with distinction in a raid into Norway and was later sent to North Africa, where there was little action at the time. He resumed drinking and was debilitated by an enlarged liver complicated by a resurgence of malaria. At the end of 1940, he returned to England and was discharged from the army on health grounds on 2 May 1941, by which time he had once again reached the rank of captain. Kermit had appealed this discharge all the way to the British Prime Minister, Winston Churchill. Churchill, after reviewing his record, upheld the medical discharge.

When he returned to the US, he turned to drinking to forget his problems. So worried was his wife about his deteriorating condition, she went to the extreme of seeking the help of Kermit's cousin, President Franklin D. Roosevelt, who ordered the FBI to track him down and he was brought back to his family. The President thought it would be best if he were moved as far as possible from some of his friends who were such a negative influence on him, gave him a commission as a major in the United States Army, and had him transferred and posted to Fort Richardson, Alaska, where he worked as an intelligence officer and helped establish a territorial militia of Eskimos and Aleuts.

Kermit's paternal grandmother, Theodore Roosevelt's mother Martha Bulloch Roosevelt, had led a life with supreme highs as well as debilitating lows. His paternal uncle, Elliott Roosevelt, was afflicted with chronic bouts of depression and died of alcoholism and drug abuse. His maternal grandfather, meanwhile, had been an alcoholic. Alcoholism plagued Kermit much of his adult life.

While there were some similarities between Kermit and his Uncle Elliot throughout both their lives, Kermit's dreamy nature was often covered up by an unyielding drive when presented with a task or goal. When motivated, his efforts, particularly during the descent of the River of Doubt, were almost superhuman. Though he always enjoyed a drink, his problems truly arose when he failed to transition the skills that made him an exceptional adventurer to his daily life in the civilized world. His situation worsened when he learned of his father's death while fighting overseas. His failure to live up to his talents was tragedy for all his friends and family.

In Alaska, far from home, Kermit continued to fight his lifelong battle with chronic depression and alcohol. He committed suicide on June 4, 1943, by a self-inflicted gunshot. He was discovered in his house with a gunshot wound in the head by Dr. Sanford Couch Monroe who later filed the autopsy report. His death was reported to his mother, Edith Roosevelt, whose favorite son he had been, as a heart attack. Given the sensitive nature of his death, for many years the cause of death continued to be described as heart disease. Only in later years did the true circumstances of his death become known. He was interred in Fort Richardson National Cemetery near Anchorage, where a memorial stone gateway was erected in his honor in 1949.

He was survived by his wife Belle and four children: Kermit "Kim" Roosevelt, Jr., Joseph Willard Roosevelt, Belle Wyatt Roosevelt, and Dirck Roosevelt.

The town of Kermit, Texas was named for him (he had visited Winkler County, Texas a few months earlier to hunt antelope). The town of Kermit, West Virginia is also named after him. Finally, the Luzon-class repair ship USS Kermit Roosevelt was named in his honor.

The story of Kermit's life becomes sadder when one considers that his greatest driving force on the descent of the River of Doubt, where his star shined brightest, was his father's decision to spare his own life rather than commit suicide. Few people knew that on all his adventures Roosevelt packed a lethal dose of morphine. After his initial leg injury, he planned to down the vial thinking that he would save the group the trouble of carrying a sick man. To his horror, his son vowed to carry him out alive or dead and it became a pact between the two that they would walk out together. His father's choice to live was Kermit's greatest inspiration but he ultimately could not save himself.