Jinnah was the first of seven children of Jinnahbhai, a prosperous merchant. After being taught at home, Jinnah was sent to the Sind Madrasasah High School in 1887. Later he attended the Mission High School, where, at the age of 16, he passed the matriculation examination of the University of Bombay. On the advice of an English friend, his father decided to send him to England to acquire business experience. Jinnah, however, had made up his mind to become a barrister. In keeping with the custom of the time, his parents arranged for an early marriage for him before he left for England.

In London he joined Lincoln's Inn, one of the legal societies that prepared students for the bar. In 1895, at the age of 19, he was called to the bar. While in London Jinnah suffered two severe bereavements—the deaths of his wife and his mother. Nevertheless, he completed his formal studies and also made a study of the British political system, frequently visiting the House of Commons. He was greatly influenced by the liberalism of William E. Gladstone (Gladstone, William Ewart), who had become prime minister for the fourth time in 1892, the year of Jinnah's arrival in London. Jinnah also took a keen interest in the affairs of India and in Indian students. When the Parsi leader Dadabhai Naoroji (Naoroji, Dadabhai), a leading Indian nationalist, ran for the English Parliament, Jinnah and other Indian students worked day and night for him. Their efforts were crowned with success, and Naoroji became the first Indian to sit in the House of Commons.

When Jinnah returned to Karāchi in 1896, he found that his father's business had suffered losses and that he now had to depend on himself. He decided to start his legal practice in Bombay, but it took him years of work to establish himself as a lawyer.

It was nearly 10 years later that he turned toward active politics. A man without hobbies, his interest became divided between law and politics. Nor was he a religious zealot: he was a Muslim in a broad sense and had little to do with sects. His interest in women was also limited to Ruttenbai—the daughter of Sir Dinshaw Petit, a Bombay Parsi millionaire—whom he married over tremendous opposition from her parents and others. The marriage proved an unhappy one. It was his sister Fatima who gave him solace and company.

Jinnah first entered politics by participating in the 1906 Calcutta session of the Indian National Congress, the party that called for dominion status and later for independence for India. Four years later he was elected to the Imperial Legislative Council—the beginning of a long and distinguished parliamentary career. In Bombay he came to know, among other important Congress personalities, Gopal Krishna Gokhale (Gokhale, Gopal Krishna), the eminent Maratha leader. Greatly influenced by these nationalist politicians, Jinnah aspired during the early part of his political life to become “a Muslim Gokhale.” Admiration for British political institutions and an eagerness to raise the status of India in the international community and to develop a sense of Indian nationhood among the peoples of India were the chief elements of his politics. At that time, he still looked upon Muslim interests in the context of Indian nationalism.

But, by the beginning of the 20th century, the conviction had been growing among the Muslims that their interests demanded the preservation of their separate identity rather than amalgamation in the Indian nation that would for all practical purposes be Hindu. Largely to safeguard Muslim interests, the All-India Muslim League was founded in 1906. But Jinnah remained aloof from it. Only in 1913, when authoritatively assured that the league was as devoted as the Congress to the political emancipation of India, did Jinnah join the league. When the Indian Home Rule League was formed, he became its chief organizer in Bombay and was elected president of the Bombay branch.

“Ambassador of Hindu–Muslim unity.” Jinnah's endeavours to bring about the political union of Hindus and Muslims earned him the title of “the best ambassador of Hindu–Muslim unity,” an epithet coined by Gokhale. It was largely through his efforts that the Congress and the Muslim League began to hold their annual sessions jointly, to facilitate mutual consultation and participation. In 1915 the two organizations held their meetings in Bombay and in 1916 in Lucknow, where the Lucknow Pact was concluded. Under the terms of the pact, the two organizations put their seal to a scheme of constitutional reform that became their joint demand vis-à-vis the British government. There was a good deal of give and take, but the Muslims obtained one important concession in the shape of separate electorates, already conceded to them by the government in 1909 but hitherto resisted by the Congress.

Meanwhile, a new force in Indian politics had appeared in the person of Mohandas K. Gandhi (Gandhi, Mohandas Karamchand). Both the Home Rule League and the Indian National Congress had come under his sway. Opposed to Gandhi's Non-cooperation Movement and his essentially Hindu approach to politics, Jinnah left both the League and the Congress in 1920. For a few years he kept himself aloof from the main political movements. He continued to be a firm believer in Hindu–Muslim unity and constitutional methods for the achievement of political ends. After his withdrawal from the Congress, he used the Muslim League platform for the propagation of his views. But during the 1920s the Muslim League, and with it Jinnah, had been overshadowed by the Congress and the religiously oriented Muslim Khilāfat committee.

When the failure of the Non-cooperation Movement and the emergence of Hindu revivalist movements led to antagonism and riots between the Hindus and Muslims, the league gradually began to come into its own. Jinnah's problem during the following years was to convert the league into an enlightened political body prepared to cooperate with other organizations working for the good of India. In addition, he had to convince the Congress, as a prerequisite for political progress, of the necessity of settling the Hindu–Muslim conflict.

To bring about such a rapprochement was Jinnah's chief purpose during the late 1920s and early 1930s. He worked toward this end within the legislative assembly, at the Round Table Conferences in London (1930–32), and through his 14 points, which included proposals for a federal form of government, greater rights for minorities, one-third representation for Muslims in the central legislature, separation of the predominantly Muslim Sind region from the rest of the Bombay province, and the introduction of reforms in the North-West Frontier Province. But he failed. His failure to bring about even minor amendments in the Nehru Committee proposals (1928) over the question of separate electorates and reservation of seats for Muslims in the legislatures frustrated him. He found himself in a peculiar position at this time; many Muslims thought that he was too nationalistic in his policy and that Muslim interests were not safe in his hands, while the Indian National Congress would not even meet the moderate Muslim demands halfway. Indeed, the Muslim League was a house divided against itself. The Punjab Muslim League repudiated Jinnah's leadership and organized itself separately. In disgust, Jinnah decided to settle in England. From 1930 to 1935 he remained in London, devoting himself to practice before the Privy Council. But when constitutional changes were in the offing, he was persuaded to return home to head a reconstituted Muslim League.

Soon preparations started for the elections under the Government of India Act of 1935. Jinnah was still thinking in terms of cooperation between the Muslim League and the Hindu Congress and with coalition governments in the provinces. But the elections of 1937 proved to be a turning point in the relations between the two organizations. The Congress obtained an absolute majority in six provinces, and the league did not do particularly well. The Congress decided not to include the league in the formation of provincial governments, and exclusive all-Congress governments were the result. Relations between Hindus and Muslims started to deteriorate, and soon Muslim discontent became boundless.

Jinnah had originally been dubious about the practicability of Pakistan, an idea that Sir Muḥammad Iqbāl had propounded to the Muslim League conference of 1930; but before long he became convinced that a Muslim homeland on the Indian subcontinent was the only way of safeguarding Muslim interests and the Muslim way of life. It was not religious persecution that he feared so much as the future exclusion of Muslims from all prospects of advancement within India as soon as power became vested in the close-knit structure of Hindu social organization. To guard against this danger he carried on a nationwide campaign to warn his coreligionists of the perils of their position, and he converted the Muslim League into a powerful instrument for unifying the Muslims into a nation.

At this point, Jinnah emerged as the leader of a renascent Muslim nation. Events began to move fast. On March 22–23, 1940, in Lahore, the league adopted a resolution to form a separate Muslim state, Pakistan. The Pakistan idea was first ridiculed and then tenaciously opposed by the Congress. But it captured the imagination of the Muslims. Pitted against Jinnah were men of the stature of Gandhi and Jawaharlal Nehru. And the British government seemed to be intent on maintaining the political unity of the Indian subcontinent. But Jinnah led his movement with such skill and tenacity that ultimately both the Congress and the British government had no option but to agree to the partitioning of India. Pakistan thus emerged as an independent state in 1947.