Advani joined the Rashtriya Swayamsevak Sangh (RSS) in 1942. He became a pracharak (full-time worker) of the Karachi branch and developed several shakhas there. After Partition, Advani was sent as a pracharak to Matsya-Alwar in Rajasthan, which had witnessed communal violence following Partition. He worked in Alwar, Bharatpur, Kota, Bundi and Jhalawar districts until 1952.

Advani became a member of the Bharatiya Jana Sangh, also known simply as the Jana Sangh, a political party founded in 1951 by Syama Prasad Mookerjee in collaboration with the RSS. He was appointed as the secretary to S. S. Bhandari, then General Secretary of the Jana Sangh in Rajasthan. In 1957, he was moved to Delhi to look after the Parliamentary affairs. He soon became the General Secretary and, later, President of the Delhi unit of the Jana Sangh. After the 1967 elections, he became the leader of the city's Metropolitan Council. He also assisted K. R. Malkani in editing the RSS weekly Organiser, and became a member of the national executive in 1966.

He became member of the Rajya Sabha from Delhi for the six-year tenure from 1970. After serving various positions in the Jana Sangh, he became its President in 1973 at the Kanpur session of the party working committee. His first act as President of the BJS was to expel founder member and veteran leader Balraj Madhok from primary membership of the party for supposedly violating the party directives and acting against the interests of the party. He was a Rajya Sabha member from Gujarat from 1976 to 1982. After the Indira Gandhi's Emergency, the Jana Sangh and many other opposition parties merged into the Janata Party. Advani and colleague Atal Bihari Vajpayee fought the Lok Sabha Elections of 1977 as members of the Janata Party.

The Janata Party was formed by political Leaders and Activists of various political parties who had been united in opposing the state of Emergency imposed in 1975 by then-Prime Minister Indira Gandhi. After elections were called in 1977, the Janata Party was formed from the union of the Congress (O), the Swatantra Party, the Socialist Party of India, the Jana Sangh and the Lok Dal. Jagjivan Ram split from the Indian National Congress, bringing a small faction known as the Congress for Democracy with him, and joined the Janata alliance. The widespread unpopularity of Emergency rule gave the Janata Party and its allies a landslide victory in the election. Morarji Desai became the Prime Minister of India, Advani became the Minister of Information and Broadcasting and Vajpayee became the Foreign Minister.

Under Advani, the BJP became the political face of the Ram Janmabhoomi campaign. In the early 1980s, the Vishwa Hindu Parishad (VHP) had begun a movement for the construction of a temple dedicated to the Hindu deity Rama at the site of the Babri Masjid in Ayodhya. The agitation was on the basis of the belief that the site was the birthplace of Rama, and that a temple once stood there that had been demolished by the Mughal Emperor Babur when he constructed the Babri mosque. The Archaeological Survey of India (ASI) has supported the claim that a Hindu structure once stood at the site, without commenting on a possible demolition. The BJP threw its support behind this campaign, and made it a part of their election manifesto, which provided rich dividends in the general elections of 1989. Despite the Congress winning a plurality in the election, it declined to form a government, and so the National Front government of VP Singh was sworn in. The support of the BJP, with its tally of 86 seats, was crucial to the new government.

The erstwhile members of the Jana Sangh, quit the Janata Party and they formed the new Bharatiya Janata Party. Advani became a prominent leader of the newly founded BJP and represented the party in the Rajya Sabha (upper house of the Indian Parliament) from Madhya Pradesh for two terms beginning in 1982.

Advani embarked on a "Rath Yatra," or chariot journey, to mobilise karsevaks, or volunteers, to converge upon the Babri Masjid to offer prayers. This Rath Yatra, undertaken in an air-conditioned van decorated to look like a chariot, began from Somnath in Gujarat and covered a large portion of Northern India until it was stopped by the Chief Minister of Bihar, Laloo Prasad Yadav, on the grounds that it was leading to communal violence. In the 1991 general elections, the BJP won the second largest number of seats, after the Congress.

In 1992, two years after Advani ended his yatra, despite assurances given by the Kalyan Singh led BJP Government to the Supreme Court, the Babri Masjid was demolished by the communal forces alleged complicity of the Kalyan Singh government. Advani is one of the main accused in the Babri Masjid case.

After the fall of two United Front government between 1996 and 1998 (H. D. Deve Gowda and I. K. Gujral), the Lok Sabha, (lower house) of India's Parliament was dissolved and new elections were held. Now, a coalition of political parties signed up with BJP to form the Nationwide Democratic Alliance (NDA), headed by A. B. Vajpayee. The NDA won a majority of seats in parliament. However, the govt survived only 13 several months until mid-1999 when All Indian Anna Dravida Munnetra Kazhagam (AIADMK) under J. Jayalalitha withdrew its assistance to govt. With the NDA no longer having a majority, India's Parliament was again dissolved and new elections were organised. Vajpayee remained the Prime Minister until elections were organised.

After two years in the political wilderness, the BJP-led National Democratic Alliance (NDA), came to power with Vajpayee returning as Prime Minister in March 1998, when elections were called after India saw two unstable Governments headed by H. D. Deve Gowda and I. K. Gujral respectively.

Vajpayee retired from active politics after the 2004 defeat, putting Advani to the forefront of the BJP. Advani became Leader of Opposition in the Lok Sabha from 2004 to 2009. During this period, Advani had to deal with rebellion from within the party. His two close associates, Uma Bharati, and Madan Lal Khurana, and longtime rival Murali Manohar Joshi publicly spoke out against him. In June 2005, he drew much criticism when he, while on a visit to the Jinnah Mausoleum at Karachi – his town of birth, endorsed Mohammad Ali Jinnah and described him a "secular" leader. This did not sit well with the RSS either and Advani was forced to relinquish his post as BJP President. However, he withdrew the resignation a few days later.

The relationship between Advani and the RSS reached a low point when the latter's chief K. S. Sudarshan opined that both Advani and Vajpayee give way to new Leaders. At the Silver Jubilee celebrations of the BJP in Mumbai in December 2005, Advani stepped down as party President and Rajnath Singh, a relatively junior Politician from the state of Uttar Pradesh was elected in his place. In March 2006, following a bomb blast at a Hindu shrine at Varanasi, Advani undertook a "Bharat Suraksha Yatra" (Sojourn for National Security), to highlight the alleged failure of the ruling United Progressive Alliance in combating terrorism.

A major factor in favour of Advani was that he had always been the most powerful leader in the BJP with the exception of Vajpayee, who endorsed Advani's candidacy. On 2 May 2007, BJP President Rajnath Singh stated that: "After Atal there is only Advani. Advani is the natural choice. It is he who should be PM". On 10 December 2007, the Parliamentary Board of BJP formally announced that L. K. Advani would be its prime ministerial candidate for the general elections due in 2009.

My Country My Life is an autobiographical book by L. K. Advani. The book was released on 19 March 2008 by Abdul Kalam, the eleventh President of India. The book has 1,040 pages and narrates autobiographical accounts and events in the life of Advani. The book became a best seller in the non-fiction category. The book website claims that more than 1,000,000 copies have been sold. The book includes mentions of events in Indian politics and India's history from 1900 till 2007.

However, Indian National Congress and its allies won the 2009 general elections, allowing incumbent Prime Minister Manmohan Singh to continue in office. Following the defeat in the elections, L. K. Advani paved the way for Sushma Swaraj to become the Leader of Opposition in the Lok Sabha.

Advani unexpectedly resigned from all his posts in the BJP on 10 June 2013 following the appointment of Narendra Modi as the head of the electoral campaign of BJP for the 2014 elections on 9 June 2013. He rued that the BJP was no longer the "same idealistic party" created by Shyama Prasad Mookerjee, Deendayal Upadhyaya, Nanaji Deshmukh and Atal Bihari Vajpayee. The BJP senior committee rejected his resignation on 10 June 2013. Ultimately, Advani withdrew his resignation on 11 June 2013 after Rajnath Singh (BJP President) assured Advani that his concerns about the functioning of the Party would be properly addressed.

In 2014, Advani was dropped from the BJP Parliamentary Board and included in the Marg Darshak Mandal of the BJP along with Murli Manohar Joshi and Atal Bihari Vajpayee.

L K Advani married Kamla Advani (1932-2016) in February 1965. He has a son, Jayant, and a daughter, Pratibha. Pratibha Advani produces TV serial shows, and also supports her father in his political activities.His wife died on 6 April 2016 due to old age.

In April 2017, Supreme Court of India reinstated a case of Criminal conspiracy against LK Advani and other BJP Leaders.He is considered the Architect of Ram Janmabhoomi movement