Born in Pune, Thackeray began his professional career as a cartoonist with the English language daily The Free Press Journal in Mumbai but left it in 1960 to form his own political weekly Marmik. His political philosophy was largely shaped by his father Keshav Sitaram Thackeray, a leading figure in the Samyukta Maharashtra movement (United Maharashtra movement), which advocated the creation of a separate linguistic state of Maharashtra. Through Marmik, he campaigned against the growing influence of Gujaratis, Marwaris, and southern Indians in Mumbai. In 1966, Thackeray formed the Shiv Sena party to advocate more strongly the place of Maharashtrians in Mumbai's political and professional landscape. In the late 1960s and early 1970s, Thackeray built the party by forming temporary alliances with nearly all of Maharashtra's political parties.
Thackeray was criticised for his praise of Adolf Hitler which he later neither admitted nor denied. He was quoted by Asiaweek as saying: "I am a great admirer of Hitler, and I am not ashamed to say so! I do not say that I agree with all the methods he employed, but he was a wonderful organiser and orator, and I feel that he and I have several things in common...What India really needs is a dictator who will rule benevolently, but with an iron hand." In a 1993 interview, Thackeray stated, "There is nothing wrong" if "Muslims are treated as Jews were in Nazi Germany." In another 1992 interview, Thackeray stated, "If you take Mein Kampf and if you remove the word 'Jew' and put in the word 'Muslim', that is what I believe in". Indian Express published an interview on 29 January 2007: "Hitler did very cruel and ugly things. But he was an artist, I love him [for that]. He had the power to carry the whole nation, the mob with him. You have to think what magic he had. He was a miracle...The killing of Jews was wrong. But the good part about Hitler was that he was an artist. He was a daredevil. He had good qualities and bad. I may also have good qualities and bad ones."
He was supportive of the Sri Lankan Liberation Tigers of Tamil Eelam.
Thackeray also declared that he was "not against every Muslim, but only those who reside in this country but do not obey the laws of the land...I consider such people [to be] traitors." The Shiv Sena is viewed by the liberal media as being anti-Muslim, though Shiv Sena members officially reject this accusation. When explaining his views on Hindutva, he conflated Islam with violence and called on Hindus to "fight terrorism and fight Islam." In an interview with Suketu Mehta, he called for the mass expulsion of illegal Bangladeshi Muslim migrants from India and for a visa system to enter Mumbai, the Indian National Congress state government had earlier during the Indira Gandhi declared national emergency considered a similar measure.
He told India Today "[Muslims] are spreading like a cancer and should be operated on like a cancer. The...country should be saved from the Muslims and the police should support them [Hindu Maha Sangh] in their struggle just like the police in Punjab were sympathetic to the Khalistanis." However, in an interview in 1998, he said that his stance had changed on many issues that the Shiv Sena had with Muslims, particularly regarding the Babri Mosque or Ram Janmabhoomi issue: "We must look after the Muslims and treat them as part of us." He also expressed admiration for Muslims in Mumbai in the wake of the 11 July 2006 Mumbai train bombings perpetrated by Islamic fundamentalists. In response to threats made by Abu Azmi, a leader of the Samajwadi Party, that accusations of terrorism directed at Indian Muslims would bring about communal strife, Thackeray said that the unity of Mumbaikars (residents of Mumbai) in the wake of the attacks was "a slap to fanatics of Samajwadi Party leader Abu Asim Azmi" and that Thackeray "salute[s] those Muslims who participated in the two minutes' silence on July 18 to mourn the blast victims." Again in 2008 he wrote: "Islamic terrorism is growing and Hindu terrorism is the only way to counter it. We need suicide bomb squads to protect India and Hindus." He also reiterated a desire for Hindus to unite across linguistic barriers to see "a Hindustan for Hindus" and to "bring Islam in this country down to its knees."
In 2008, following agitation against Biharis and other north Indians travelling to Maharashtra to take civil service examinations for the Indian Railways due to an overlimit of the quota in their home provinces, Thackeray also said of Bihari MPs that they were "spitting in the same plate from which they ate" when they criticised Mumbaikars and Maharashtrians. He wrote: "They are trying to add fuel to the fire that has been extinguished, by saying that Mumbaikars have rotten brains." He also criticised Chhath Puja, a holiday celebrated by Biharis and those from eastern Uttar Pradesh, which occurs on six days of the Hindu month of Kartik. He said that it was not a real holiday. This was reportedly a response to MPs from Bihar who had disrupted the proceedings of the Lok Sabha in protest to the attacks on North Indians. Bihar Chief Minister Nitish Kumar, upset with the remarks, called on the prime minister and the central government to intervene in the matter. A Saamna editorial prompted at least 16 MPs from Bihar and Uttar Pradesh, belonging to the Rashtriya Janata Dal, Janata Dal (United), Samajwadi Party and the Indian National Congress, to give notice for breach of privilege proceedings against Thackeray. After the matter was raised in the Lok Sabha, Speaker Somnath Chatterjee said: "If anybody has made any comment on our members' functioning in the conduct of business in the House, not only do we treat that with the contempt that it deserves, but also any action that may be necessary will be taken according to procedure and well established norms. Nobody will be spared.'"
In 2009, he criticised Indian cricketer Sachin Tendulkar, a "Marathi icon", for saying he was an Indian before he was a Maharashtrian.