There are myths and legends that say Hongli was actually a Han and not of Manchu descent, while there were some that say Hongli was only half Manchu and half Han Chinese descent. Nevertheless, looking at historical records, Hongli was adored both by his grandfather, the Kangxi Emperor and his father, the Yongzheng Emperor. Some historians argue that the main reason why Kangxi Emperor appointed Yongzheng as his successor was because Qianlong was his favourite grandson. He felt that Hongli's mannerisms were very close to his own. As a teenager he was very capable in martial arts, and possessed a high literary ability.

After his father's succession in 1722, Hongli became the Prince Bao. Like many of his uncles, Hongli entered into a battle of succession with his older half-brother Hongshi, who had the support of a large faction of court officials, as well as Yinsi, the Prince Lian. For many years the Yongzheng Emperor did not appoint anyone to the position of Crown Prince, but many in court speculated his favoring of Hongli. Hongli went on inspection trips to the south, and was known to be an able negotiator and enforcer. He was also chosen as chief regent on occasions, when his father was away from the capital. Even before Hongli's succession was read out to the assembled court, it was widely known who the new emperor would be. The young Hongli had been a favorite of his grandfather, Kangxi, and his father alike; Yongzheng had entrusted a number of important ritual tasks to him while Hongli was still a prince, and included him in important court discussions of military strategy. Hoping to avoid repetition of the succession crisis that had tainted his own accession to the throne, he had the name of his successor placed in a sealed box secured behind the tablet over the throne in the Palace of Heavenly Purity (Qianqing Gong). The name in the box was to be revealed to other members of the imperial family in the presence of all senior ministers only upon the death of the Emperor. Yongzheng died suddenly in 1735, the will was taken out and read out before the entire Qing Court, and Hongli became the 4th Manchu Emperor of China. He took the Calender Name of Qianlong, 乾 means heaven, 隆 means prosperity, which mean "Heavenly Prosperity".

The Qianlong Emperor was a successful military leader, presiding over a large expansion of the territory controlled by the Qing dynasty. This was made possible not only by Qing strength, but also by the disunity and declining strength of the Inner Asian peoples. Under Qianlong, Chinese Turkestan was incorporated into the Qing dynasty's rule and renamed Xinjiang, while to the West, Ili was conquered and garrisoned. The incorporation of Xinjiang into the Qing empire resulted from the final defeat and destruction of the Dzungars (or Zunghars), a coalition of Western Mongol tribes. Between 1755 and 1758, about 80% of the Dzungar population, or between 480,000 and 800,000 people, were killed in what "amounted to the complete destruction of not only the Zunghar state but of the Zunghars as a people." Although, according to a nineteenth-century Chinese estimate, as much as 40% of the Dzungar population may have been killed by smallpox, historian Peter Perdue has shown that the decimation of the Dzungars was the result of an explicit policy of extermination launched by the Qianlong emperor.

Throughout this period there were continued Mongol interventions in Tibet and a reciprocal spread of Tibetan Buddhism in Mongolia. Qianlong again sent armies into Tibet and firmly established the Dalai Lama as ruler, with a Qing resident and garrison to preserve Chinese sovereignty. Further afield, military campaigns against Nepalese, and Gurkhas forced these peoples to submit and send tribute.

The Qing Myanmar campaign (1765–1770) was the most disastrous frontier war that the Qing dynasty had ever waged. In the beginning, The Emperor had envisaged winning this war in one easy stroke, as he deemed Myanmar no more than a remote barbarian tribe without any power. After the Green Standard troops in Yunnan failed to bring the Myanmar to their knees, Qianlong sent his elite Manchu troops in. A regional conflict was thus escalated into a major frontier war that involved military maneuvers nationwide. At the front, the Manchu Bannermen had to deal with the unfamiliar tropical jungles and swamps, and above all, the lethal endemic diseases. Not only did one after another commander-in-chief of the Qing dynasty fail to conquer Myanmar, but the Qing troops also suffered extremely heavy casualties. After a gruelling four-year campaign, a truce was reached by the field commanders of the two sides at the end of 1769 with the Qing invading expedition failing to conquer Myanmar and withdrawing in disarray. To rehabilitate itself, the Qing dynasty kept a heavy military lineup in the border areas of Yunnan for about one decade in an attempt to wage another war while imposing a ban on inter-border trade for two decades.

The circumstances in Vietnam were not as successful. In 1787 the last Le king Le Chieu Thong fled Vietnam and formally requested that he be restored to his throne in Thanglong (Hanoi today). The Qianlong Emperor agreed and sent a large army into Vietnam to remove the Tay Son (peasant rebels who had captured all of Vietnam). The capital, Thanglong, was conquered in 1788 but a few months later, the Chinese army was defeated and the invasion turned into a debacle due to the surprise attack during Tết by Nguyen Hue, the second and most capable of the three Tay Son brothers. The Chinese gave formal protection to the Le emperor and his family, and would not intervene in Vietnam for another 90 years.

Overall the Qianlong Emperor's military expansion captured millions of square miles and brought into the empire non-Han-Chinese peoples — such as Uyghurs, Kazakhs, Kyrgyzs, Evenks and Mongols -- who were potentially hostile. It was also a very expensive enterprise; the funds in the Imperial Treasury were almost all put into military expeditions. This may have been the cause of the later decline of the dynasty — when faced with a Western threat, the army was unable to develop and upgrade its weapons. Though the wars were successful, they were not overwhelmingly so. The army declined noticeably and had a difficult time facing some enemies: the Jin Chuan area took 2–3 years to conquer — at first the Qing army were mauled, though Yue Zhongqi later took control of the situation. The battle with the Dzungars was closely fought, and caused heavy losses on both sides. At the end of the frontier wars, the army had started to weaken significantly. In addition to a more lenient military system, warlords became satisfied with their lifestyles. Since most of the warring had taken place, warlords no longer saw any reason to train their armies, resulting in a rapid military decline by the end of Qianlong's reign. This is the main reason for the military's failure against the White Lotus Sect, at the very end of Qianlong's years.

The Qianlong Emperor was also a major patron of the arts. The most significant of his commissions was a catalogue of all important works on Chinese culture, the Siku Quanshu. Produced in 36,000 volumes, containing about 3450 complete works and employing as many as 15,000 copyists, the entire work took some twenty years. It preserved numerous books, but was also intended as a way to ferret out and suppress political opponents. Some 2,300 works were listed for total suppression and another 350 for partial suppression. The aim was to destroy the writings that were anti-Qing or rebellious, that insulted previous barbarian dynasties, or that dealt with frontier or defense problems.

The Qianlong Emperor was a passionate poet and essayist. In his collected writings, which were published in a tenfold series between 1749 and 1800, over 40,000 poems and 1,300 prose texts are listed, making him one of the most prolific writers of all time. There is a long tradition of poems of this sort in praise of particular objects (yongwu shi), and the Qianlong Emperor used it in order to link his name both physically and intellectually with ancient artistic tradition. He also regularly added poetic inscriptions to the paintings of the imperial collection, following the example of the emperors of the Song dynasty and the literati painters of the Ming. They were a mark of distinction for the work, and a visible sign of his rightful role as Emperor. Most particular to the Qianlong Emperor is another type of inscription, revealing a unique practice of dealing with works of art that he seems to have developed for himself. On certain fixed occasions over a long period he contemplated a number of paintings or works of calligraphy which possessed special meaning for him, inscribing each regularly with mostly private notes on the circumstances of enjoying them, using them almost as a diary. They became part of his life and he took them with him on his travels in order to compare paintings with the actual landscape, or to hang them in special rooms in palaces where he lodged, to inscribe them on every visit there.

Pottery, ceramics and particularly applied arts, such as enameling, metal work and lacquer work flourished during his reign; a substantial part of his collection is in the Percival David Foundation in London. The Victoria and Albert Museum and The British Museum also have good collections of Qianlong reign Art.

The full editing of Siku Quanshu was completed in about ten years, during these ten years, 3100 titles (or works), about 150,000 copies of books were either burnt or banned. Of those volumes that had been categorized into Siku Quanshu, many were subjected to deletion and modification. Books published during the Ming dynasty suffered the greatest damage. Qianlong Emperor used literary inquisition to silence opposition. The accusation of individuals began with the authority's own interpretation of the true meaning of the corresponding words, the necessary 'evidence' needed to achieve a successful persecution would come from further interpretation of the words. The authority would judge any single character or any single sentence's neutrality; if the authority had decided these words, or sentence were derogative or cynic towards the rulers, then persecution would begin. In Qianlong's time, there were 53 cases of literary inquisition, resulting in the victims being beheaded, or corpses being mutilated, or victims being slowly sliced into pieces until death (Lingchi).

Architecturally, Qianlong took personal interest in the expansion of the Old Summer Palace and commissioned the Italian Jesuit Giuseppe Castiglione for the construction of the Xiyanglou, or the Western-style mansion, to satisfy his taste for exotic buildings and objects. He also commissioned the French Jesuit Michel Benoist, to design a series of timed waterworks and fountains complete with underground machinery and pipes, for the amusement of the Imperial family. The French Jesuit Jean Denis Attiret also became "Painter to the Emperor" Qianlong. During his reign the Emin Minaret was built in Turpan to commemorate his father.

In his later years, Qianlong was spoiled with power and glory, becoming disillusioned and complacent in his reign, placing his trust in corrupt officials like Yu Ming Zhong, and later Heshen. As Heshen was the highest ranked minister and most favoured by Qianlong at the time, the day-to-day governance of the country was left in his hands, while Qianlong himself indulged in the arts, luxuries and literature. When Heshen was executed it was found that his personal fortune exceeded that of the country's depleted treasury, amount to 900,000,000 taels of silver, the total of 12 years of Treasury surplus of Manchu Qing court. Qianlong began his reign with about 33,950,000 taels of silver in Treasury surplus. At the peak of Qianlong's reign, around 1775, even with further tax cuts, the treasury surplus still reached 73,900,000 taels, a record unmatched by his predecessors, Kangxi or Yongzheng both of whom had implemented remarkable tax cut policies. However, due to numerous factors such as long term embezzlement and corruption by officials, frequent expeditions South, huge palace constructions, many war and rebellion campaigns as well as his own extravagant lifestyle, all of these cost the treasury a total of 150,200,000 silver taels. This, coupled with his senior age and the lack of political reforms, ushered the beginning of the gradual decline and eventual demise of the Qing dynasty and empire, casting a shadow over his glorious and brilliant political life.

During the mid-eighteenth century, Qianlong began to face pressures from the West to increase foreign trade. The proposed cultural exchange between the British Empire at the time and the Qing Empire collapsed due to many factors. Firstly, there was a lack of any precedent interaction with overseas foreign kingdoms apart from neighbouring tributory states to guide Qianlong towards a more informed response. Furthermore, competing worldviews that were incompatible between China and Britain, the former holding entrenched beliefs that China was the "central kingdom", and the latter's push for rapid liberalization of trade relations, worsened ties. George Macartney, was sent by King George III as ambassador extraordinary to seek a range of trade concessions. He was granted an audience with the Qianlong Emperor, and attended the Emperor's 80th birthday. There is continued discussion about the nature of the audience, and what level of ceremonials were performed. Demands from the Qing Court that the British Trade ambassadors kneel and perform the kowtow were strongly resisted by Macartney, and debate continues as to whether or not this occurred, opinions being offered by Qing courtiers and British delegates alike. A description of the Emperor is provided in the account of one of the visiting Englishmen, Aeneas Anderson:

The Emperor is about five feet ten inches in height, and of a slender but elegant form; his complexion is comparatively fair, though his eyes are dark; his nose is rather aquiline, and the whole of his countenance presents a perfect regularity of feature, which, by no means, announce the great age he is said to have attained; his person is attracting, and his deportment accompanies by an affability, which, without lessening the dignity of the prince, evinces the amiable character of the man. His dress consisted of a loose robe of yellow silk, a cap of black velvet with a red ball on the top, and adorned with a peacock's feather, which is the peculiar distinction of mandarins of the first class. He wore silk boots embroidered with gold, and a sash of blue girded his waist.

It is uncertain whether Anderson actually saw the Emperor, or repeated another's sighting, as he was not involved in the ceremonies. After the reign of Qianlong, China failed to keep in pace with Western technology, allowing the Western nations to become superior to China both through economy and military. This is especially evident when Chinese forces, though far more superior in numbers and enjoyed home advantage, were no match to the much smaller Western forces, who were much better equipped with the latest weapons during the Opium wars.

To many historians, the Qianlong reign marked the beginning of the downfall of the Qing dynasty, and due to this, many historians prefer to acknowledge the rise of the Qing dynasty more as achievements of Qianlong's father and grandfather. It must be noted that Kangxi, Qianlong's grandfather, had great interest in Western technology, especially in Mathematics and Astrology, and was a much better host to Western visitors. In George Macartney's memoirs, there were many passages describing the overall poor and suffering life of inhabitants of Qing dynasty. Macartney expressed opinions which were widely disseminated:

“

The Empire of China is an old,crazy, first-rate Man of War, which a fortunate succession of and vigilant officers have contrived to keep afloat for these hundred and fifty years past, and to overawe their neighbours merely by her bulk and appearance. But whenever an insufficient man happens to have the command on deck, adieu to the discipline and safety of the ship. She may, perhaps, not sink outright; she may drift some time as a wreck, and will then be dashed to pieces on the shore; but she can never be rebuilt on the old bottom.

”

A Dutch embassy arrived to the Qianlong court in 1795, and would turn out to be the last occasion in which any European appeared before the Chinese Court within the context of traditional Chinese imperial foreign relations. Representing Dutch and Dutch East India Company interests, Isaac Titsingh traveled to Pekin in 1794-95 for celebrations of the sixtieth anniversary of the Qianlong Emperor's reign. The Titsingh delegation also included the Dutch-American Andreas Everardus van Braam Houckgeest, whose detailed description of this embassy to the Chinese court was soon after published in the U.S. and Europe. Titsingh's French translator, Chrétien-Louis-Joseph de Guignes published his own account of the Titsingh mission in 1808. Voyage a Pékin, Manille et l'Ile de France provided an alternate perspective and a useful counterpoint to other reports which were then circulating. Titsingh himself died before he could publish his version of events. In contrast to Macartney, Isaac Titsingh, the Dutch and VOC emissary in 1795 did not refuse to kowtow. In the year following Mccartney's rebuff, Titsingh and his colleagues were much feted by the Chinese because of what was construed as seemly compliance with conventional court etiquette.

In October 1795, Qianlong officially announced that in the spring of the following year he would voluntarily abdicate his throne and pass the crown to his son. It was said that Qianlong had made a promise during the year of his ascension not to rule longer than his grandfather, the Kangxi Emperor, who had reigned for 61 years. Qianlong anticipated moving out of the Hall of Mental Cultivation in the Forbidden City. These Imperial apartments had been conventionally dedicated for the exclusive use of the reigning sovereign, and the emperor ordered the beginning of construction on what was ostensibly intended as his retirement residence in another part of the Forbidden City. Despite the lavish attention devoted to these new chambers, Qianlong never moved into what was called Juanqinzhai or Studio of Exhaustion From Diligent Service. He resigned the throne at the age of 85, in the 60th year of his reign, to his son, the Jiaqing emperor at 1795. For the next four years, he held the title "Retired Emperor." He died in 1799. Despite his voluntary abdication, from 1796 to 1799 Qianlong continued to hold on to power and the Jiaqing Emperor ruled only in name.