János Áder was born into a Roman Catholic family in the small town of Csorna in Győr-Moson-Sopron County, the son of shop assistant János Áder, Sr. (1932–1980) and accountant Terézia Szabó (b. 1936), who worked for the local hospital and retired from there as deputy Director for financial affairs. Áder grew up in his hometown and completed his elementary studies there. Beginning in 1978, he studied law for five years at the faculty of Law and Political Sciences at Eötvös Loránd University in Budapest. From 1986 to 1990, he was a research fellow at the Hungarian Academy of Sciences' Sociological Research Institute.

By the late 1980s, Áder had joined the nascent Fidesz, a liberal-democratic party at the time, serving as a party legal expert. He became a member of the Opposition Round Table talks that negotiated an end to single-party rule in Hungary in 1989.

Since 1984, Áder is married to Anita Herczegh, who works as a judge. They have three daughters, Orsolya, Borbála, Júlia and a son, András.

Áder's father-in-law, Géza Herczegh, was a judge of the International Court of Justice at The Hague from 1993 to 2003.

Áder began his national political career during this transition to democracy, serving as the leader of Fidesz's national campaigns during the 1990 and 1994 parliamentary elections and as an MP from 1990 to 2009. During his twelve-year parliamentary tenure Áder served in a variety of leadership roles. From 9 September 1997 to 17 June 1998 he served as Deputy Speaker of the National Assembly, and, during the first Orbán government (18 June 1998—15 May 2002), as Speaker of the National Assembly. With Fidesz in opposition after the 2002 election, Áder led the Fidesz caucus opposition.

During the same period, Áder entered the national and international media spotlight for refusing to submit a government-sponsored revision to Act CCIV of 2011 on National Higher Education for constitutional review. The amendment, known domestically as lex CEU, impacted foreign-operating universities, including the Central European University (CEU). Áder signed the educational amendment into law on 10 April despite protest and criticism from both domestic and international Activists, scholars, and academic institutions, claiming the amendment did not seem to conflict with the constitution or international treaties. Áder's decision sparked a protest in front of the Sándor Palace, while Wikipedia pages about Áder in several languages were vandalized.

In contrast to his predecessor Schmitt, who during his presidency of almost two years did not send any bill which had been voted on by the National Assembly back for consideration by the legislature, nor submit any to the Constitutional Court for judicial review, Áder proved to be a counterweight to the Orbán government on a number of important issues. Beyond the withdrawal of election procedures bill in December 2012, Áder refused to sign the new land law which was intended to pass the right of management of national parks to the National Land Fund Managing Organization (NFA), in accordance with his environmental objectives. Áder also used his veto power in March 2016 on the controversial bill which would have declared public funds allocated to the foundations of the Hungarian National Bank (MNB) as "private funds". Left-wing liberal Journalist Gábor Miklósi (Index.hu) summarized Áder's presidency with that argument that the President remained silent on the really important matters and "his rare criticism does not hinder but legitimizes Hungary's increasingly authoritarian political system".

In June 2013, Áder apologised in the national parliament of Serbia for Hungarians' war crimes against civilian Serbs in Vojvodina in the Second World War. Some days earlier the Serbian lawmakers adopted a declaration, which condemned the massacre in Vojvodina in 1944-45 and resolutions made under the principle of collective guilt during the war. Áder said "although nobody can undo those crimes, we still believe that forgiveness following a mutual apology can point beyond reconciliation. Because we, Hungarians of today, and Serbs of today, stand united as one on the side of the innocent victims. The legacy we want to pass on to our children is that of life, justice and cooperation, and not that of death, untruth and hatred".

In December 2015, he delivered a speech during the United Nations Climate Change Conference in Paris, where he called the next 25 years as the "period when the impacts of climate change posed a direct threat to human civilization." He welcomed the ratification of the Paris Agreement, which he called a very important step for Hungary, "as the Carpathian Basin is more affected by climate change than several other European countries", he said. In June 2016, Áder wrote a letter to the heads of state of the world's top ten carbon dioxide-emitting countries to persuade them to tighten their reduction targets. Jobbik Politician Lajos Kepli accused Áder, as while he fights for the interests of environmental protection at the international level, he failed to criticize the Orbán government for their precluding domestic policy. "[Áder] sees no Problem with the taxation on solar panels, he is not bothered by the arbitrary cessation of Energy efficiency tenders [...]", Kepli argued.

János Áder has a sister Annamária (born 1960), a biology and geography high school Teacher, who married footballer and manager Gábor Pölöskei. She was appointed head of the Klebelsberg Institution Maintenance Centre (KLIK) on 1 March 2016.

Áder was re-elected for a second five-year term by the Hungarian parliament by a vote of 131–39 on 13 March 2017. His opponent was legal scholar László Majtényi, an independent candidate supported by a coalition of left-wing opposition parties. Áder thus became the second two-term post-communist head of state of Hungary (after Árpád Göncz, who served as President from 1990 to 2000). Áder took his presidential oath of office on 8 May 2017.