Baden-Powell was the son of The Reverend Baden Powell, Savilian Professor of Geometry at Oxford University and Church of England priest and his third wife, Henrietta Grace Smyth (3 September 1824 – 13 October 1914), eldest daughter of Admiral william Henry Smyth. After Powell died in 1860, to identify her children with her late husband's fame, and to set her own children apart from their half-siblings and cousins, his mother styled the family name Baden-Powell. The name was eventually legally changed by Royal Licence on 30 April 1902.

Baden-Powell had four older half-siblings from the second of his father's two previous marriages, and six full siblings Warington (1847–1921), George (1847–1898), the often-ill Augustus ("Gus") (1849–1863), Francis ("Frank") (1850–1933), Agnes (1858–1945) and Baden (1860–1937), as well as three others (Henrietta Smyth, 28 Oct 1851 – 9 March 1854; John Penrose Smyth, 21 December 1852 – 14 December 1855; and Jessie Smyth 25 November 1855 – 24 July 1856), who had all died very young before he was born.

Baden-Powell was born as Robert Stephenson Smyth Powell at 6 Stanhope Street (now 11 Stanhope Terrace), Paddington in London, on 22 February 1857. He was called Stephe (pronounced "Stevie") by his family. He was named after his godfather, Robert Stephenson, the railway and civil Engineer, and his third name was his mother's maiden name.

In 1876 Baden-Powell joined the 13th Hussars in India with the rank of lieutenant. He enhanced and honed his military scouting skills amidst the Zulu in the early 1880s in the Natal province of South Africa, where his regiment had been posted, and where he was Mentioned in Despatches. During one of his travels, he came across a large string of wooden beads. Although Baden-Powell claimed the beads had been those of the Zulu king Dinizulu, one researcher learned from Baden-Powell's diary only that he had taken beads from a dead woman's body around that time, and indeed the bead form is more similar to dowry beads than to warrior beads. The beads were later incorporated into the Wood Badge training programme he started after he founded the Scouting Movement. Baden-Powell's skills impressed his superiors and in 1890 he was brevetted Major as Military Secretary and senior Aide-de-camp to the Commander-in-Chief and Governor of Malta, his uncle General Sir Henry Augustus Smyth. He was posted to Malta for three years, also working as intelligence officer for the Mediterranean for the Director of Military Intelligence. He frequently travelled disguised as a butterfly collector, incorporating plans of military installations into his drawings of butterfly wings. In 1884 he published Reconnaissance and Scouting.

Baden-Powell was accused of illegally executing a prisoner of war in 1896, the Matabele chief Uwini, who had been promised his life would be spared if he surrendered. Uwini was sentenced to be shot by firing squad by a military court, a sentence Baden-Powell confirmed. Baden-Powell was cleared by a military court of inquiry but the colonial civil authorities wanted a civil investigation and trial. Baden-Powell later claimed he was "released without a stain on my character". Baden-Powell was also accused of allowing native African warriors under his command to massacre enemy prisoners including women, children and non-combatants.

After Rhodesia, Baden-Powell served in the Fourth Ashanti War in Gold Coast. In 1897, at the age of 40, he was brevetted colonel (the youngest colonel in the British Army) and given command of the 5th Dragoon Guards in India. A few years later he wrote a small manual, entitled Aids to Scouting, a summary of lectures he had given on the subject of military scouting, much of it a written explanation of the lessons he had learned from Burnham, to help train recruits. Using this and other methods he was able to train them to think independently, use their initiative, and survive in the wilderness.

The siege was lifted on 16 May 1900. Baden-Powell was promoted to major-general, and became a national hero. However, the British military commanders were more critical of his performance and even less impressed with his subsequent choices to again allow himself be besieged. Ultimately, his failure to properly scout the situation and abandonment of the Soldiers, mostly Australians and Rhodesians, at the Battle of Elands River led to his being removed from action.

Briefly back in the United Kingdom in October 1901, Baden-Powell was invited to visit King Edward VII at Balmoral, the monarch's Scottish retreat, and personally invested as Companion of the Order of the Bath (CB). The Order of the Bath is the fourth-most senior of the British Orders of Chivalry.

The family name legally changed from Powell to Baden-Powell by Royal Licence on 30 April 1902.

On his return from Africa in 1903, Baden-Powell found that his military training manual, Aids to Scouting, had become a best-seller, and was being used by teachers and youth organisations. Following his involvement in the Boys' Brigade as a Brigade Vice-President and Officer in charge of its scouting section, with encouragement from his friend, william Alexander Smith, Baden-Powell decided to re-write Aids to Scouting to suit a youth readership. In August 1907 he held a camp on Brownsea Island to test out his ideas. About twenty boys attended: eight from local Boys' Brigade companies, and about twelve public school boys, mostly sons of his friends.

Baden-Powell was also influenced by Ernest Thompson Seton, who founded the Woodcraft Indians. Seton gave Baden-Powell a copy of his book The Birch Bark Roll of the Woodcraft Indians and they met in 1906. The first book on the Scout Movement, Baden-Powell's Scouting for Boys was published in six instalments in 1908, and has sold approximately 150 million copies as the fourth best-selling book of the 20th century.

Boys and girls spontaneously formed Scout troops and the Scouting Movement had inadvertently started, first as a national, and soon an international phenomenon. A rally of Scouts was held at Crystal Palace in London in 1909, at which Baden-Powell met some of the first Girl Scouts. The Girl Guides were subsequently formed in 1910 under the auspices of Baden-Powell's sister, Agnes Baden-Powell. Baden-Powell's friend Juliette Gordon Low was encouraged by him to found the Girl Scouts of the USA.

In 1910, after being rebuked for a series of what were regarded as publicity gaffes, one suggesting invasion by Germany, Baden-Powell retired from the Army. Baden-Powell later claimed he was advised by King Edward VII that he could better serve his country by promoting Scouting.

Some early Scouting "Thanks" badges (from 1911) and the Scouting "Medal of Merit" badge had a swastika symbol on them. This was undoubtedly influenced by the use by Rudyard Kipling of the swastika on the jacket of his published books, including "Kim", which was used by Baden-Powell as a basis for the Wolf Cub branch of the Scouting Movement. The swastika had been a symbol for luck in India long before being adopted by the Nazis in 1920, when the National Socialist Party was still relatively unknown, and when Nazi use of the swastika became well-known, the Scouts stopped using it. According to a biography by Michael Rosenthal, Baden-Powell used the swastika because he was a Nazi sympathiser. By contrast, Jeal argues that Baden-Powell had been ignorant of the symbol's growing association with Nazism and that he had used the symbol for its centuries-old meaning of "good luck" in India. (Nazi Germany banned Scouting, a competitor to the Hitler Youth, in June 1934, seeing it as "a haven for young men opposed to the new State". Based on the regime's view of Scouting as a dangerous espionage organisation, Baden-Powell's name was included in "The Black Book", a 1940 list of people slated for detention following the planned conquest of the United Kingdom.)

In January 1912, Baden-Powell was en route to New York on a Scouting World Tour, on the ocean liner SS Arcadian, when he met Olave St Clair Soames. She was 23, while he was 55; they shared the same birthday, 22 February. They became engaged in September of the same year, causing a media sensation due to Baden-Powell's fame. To avoid press intrusion, they married in private on 30 October 1912, at St Peter's Church in Parkstone. The Scouts of England each donated a penny to buy Baden-Powell a wedding gift, a car (note that this is not the Rolls-Royce they were presented with in 1929). There is a monument to their marriage inside St Mary's Church, Brownsea Island.

On the outbreak of World War I in 1914, at the age of fifty-seven, Baden-Powell put himself at the disposal of the War Office. No command was given to him. It has been claimed that Lord Kitchener said: "he could lay his hand on several competent divisional generals but could find no one who could carry on the invaluable work of the Boy Scouts". It was rumoured that Baden-Powell was engaged in spying and Baden-Powell claimed that intelligence officers spread this myth.

In addition, when Olave's sister Auriol Davidson (née Soames) died in 1919, Olave and Robert took her three nieces, Christian (1912–1975), Clare (1913–1980), and Yvonne (1918–1995?), into their family and brought them up as their own children.

In 1927, at the Swedish National Jamboree he was awarded by the Österreichischer Pfadfinderbund with the "Großes Dankabzeichen des ÖPB.

Baden-Powell was nominated for the Nobel Peace Prize on numerous occasions, including 10 separate nominations in 1928. He was awarded the Wateler Peace Prize in 1937. In 2002, Baden-Powell was named 13th in the BBC's list of the 100 Greatest Britons following a UK-wide vote. As part of the Scouting 2007 Centenary, Nepal renamed Urkema Peak to Baden-Powell Peak.

In 1929, during the 3rd World Scout Jamboree, he received as a present a new 20-horsepower Rolls-Royce car (chassis number GVO-40, registration OU 2938) and an Eccles Caravan. This combination well served the Baden-Powells in their further travels around Europe. The caravan was nicknamed Eccles and is now on display at Gilwell Park. The car, nicknamed Jam Roll, was sold after his death by Olave Baden-Powell in 1945. Jam Roll and Eccles were reunited at Gilwell for the 21st World Scout Jamboree in 2007. Recently it has been purchased on behalf of Scouting and is owned by a charity, B-P Jam Roll Ltd. Funds are being raised to repay the loan that was used to purchase the car. Baden-Powell also had a positive impact on improvements in youth education. Under his dedicated command the world Scouting movement grew. By 1922 there were more than a million Scouts in 32 countries; by 1939 the number of Scouts was in excess of 3.3 million.

In 1931, Major Frederick Russell Burnham dedicated Mount Baden-Powell in California to his old Scouting friend from forty years before. Today their friendship is honoured in perpetuity with the dedication of the adjoining peak, Mount Burnham.

Baden-Powell's father died when he was three. Subsequently, Baden-Powell was raised by his mother, a strong woman who was determined that her children would succeed. In 1933 he said of her "The whole secret of my getting on, lay with my mother."

The Bronze Wolf Award, the only distinction of the World Organization of the Scout Movement, awarded by the World Scout Committee for exceptional services to world Scouting, was first awarded to Baden-Powell by a unanimous decision of the then International Committee on the day of the institution of the Bronze Wolf in Stockholm in 1935. He was also the first recipient of the Silver Buffalo Award in 1926, the highest award conferred by the Boy Scouts of America.

In 1937 Baden-Powell was appointed to the Order of Merit, one of the most exclusive awards in the British honours system, and he was also awarded 28 decorations by foreign states, including the Grand Officer of the Portuguese Order of Christ, the Grand Commander of the Greek Order of the Redeemer (1920), the Commander of the French Légion d'honneur (1925), the First Class of the Hungarian Order of Merit (1929), the Grand Cross of the Order of the Dannebrog of Denmark, the Grand Cross of the Order of the White Lion, the Grand Cross of the Order of the Phoenix, and the Order of Polonia Restituta.

Tim Jeal, who wrote the biography Baden-Powell, argued that Baden-Powell's distrust of communism led to his implicit support, through naïveté, of fascism. Baden-Powell admired Benito Mussolini early in the Italian fascist leader's career. In 1939 Baden-Powell noted in his diary: "Lay up all day. Read Mein Kampf. A wonderful book, with good ideas on education, health, propaganda, organisation etc. – and ideals which Hitler does not practise himself."

Baden-Powell died on 8 January 1941 and is buried at St Peter's Cemetery in Nyeri. His gravestone bears a circle with a dot in the centre "ʘ", which is the trail sign for "Going home", or "I have gone home". His wife Olave moved back to England in 1942, although when she died (in 1977), her ashes were sent to Kenya and interred beside her husband. The Kenyan government has declared Baden-Powell's grave a national monument.

Three of Baden-Powell's many biographers comment on his sexuality; the first two (in 1979 and 1986) focused on his relationship with his close friend Kenneth McLaren. Tim Jeal's later biography discusses the relationship and finds no evidence that this friendship was of an erotic nature. Jeal then examines Baden-Powell's views on women, his appreciation of the male form, his military relationships, and his marriage, concluding that, in his personal opinion, Baden-Powell was a repressed homosexual. Jeal's conclusion is disputed.

Contrary views of Baden-Powell's actions during the siege argue that his success in resisting the Boers was secured at the expense of the lives of the native African Soldiers and civilians, including members of his own African garrison. Pakenham stated that Baden-Powell drastically reduced the rations to the native garrison. However, in 2001, after subsequent research, Pakenham decidedly retreated from this position.