Timmermans previously served as First Vice-President to Jean-Claude Juncker and European Commissioner for Better Regulation, Interinstitutional Relations, the Rule of Law and the Charter of Fundamental Rights from 2014 to 2019.

On 6 January 1986, he was conscripted in the Royal Netherlands Army as a private first class for the Dutch Military Intelligence and Security Service as a Russian Prisoner of War interrogator.

On 15 March 1994, he left the Ministry of Foreign Affairs, becoming Assistant to European Commissioner Hans van den Broek.

Timmermans moved from this post a year later on 15 March 1995, to become Senior Advisor and Private Secretary to Max van der Stoel, the High Commissioner for National Minorities of the Organization for Security and Co-operation in Europe (OSCE).

After the Dutch general election of 2006 the Labour Party, Christian Democratic Appeal (CDA) and the ChristianUnion (CU) formed a coalition agreement which resulted in the formation of the Cabinet Balkenende IV.

The Treaty of Lisbon was signed whilst he was Undersecretary, before which Timmermans and Prime Minister Jan Peter Balkenende successfully lobbied to secure a greater role for national parliaments in European Union decision-making processes.

Labour politician Ronald Plasterk accused Timmermans in 2016 of doing nothing for half a year in order to acquire this responsibility, which had initially been refused because of his previous office.

In May 2014, Timmermans condemned Geert Wilders' anti-Islam sticker, saying that "The Netherlands cannot be held responsible for the adolescent behavior of a single parliamentarian.

His speech called for Dutch Prime Minister Rutte and the International Community to help bring the victims home and start an investigation to make sure that those responsible are brought to justice.

He states: "I call on the international community, on the Security Council, on anyone with influence on the situation on the ground: allow us to bring the victims’ remains home to their loved ones without any further delay.

In September 2014, the Dutch Government nominated Frans Timmermans as its prospective member of the European Commission under President-elect Jean-Claude Juncker.

[22] In May 2016, Timmermans said that Erdoğan's Turkey "has made impressive progress, particularly in recent weeks, on meeting the benchmarks of its visa liberalisation roadmap.

Timmermans said: "The new disciplinary regime undermines the judicial independence of Polish judges by not offering necessary guarantees to protect them from political control, as required by the Court of Justice of the European Union.

He said that people coming to the EU "are fellow human beings who, I think, deserve to seek refuge when they flee the barbarism that the jihadists are inflicting upon them.

[28] His party finished second in the election behind the EPP, but after having discussed the parliamentary lead candidates Manfred Weber, Timmermans and occasionally also Margrethe Vestager of the three largest European parties in the parliament several times, the European Council initially intended to nominate Timmermans for the office of commission president mostly because of Weber's alleged missing experience on the international stage.

However, when governments from Eastern Europe protested this decision because of their strong opposition against Timmerman's fight on behalf of the commission for the rule of law and against reforms which are prospected to undermine it in these countries, the Council almost unanimously proposed German Defense Minister Ursula von der Leyen as a compromise candidate in July 2019, a controversial decision among many members of the European Parliament, as she had neither been running for the office nor taken part in the European election, while such a process had informally been agreed as the gold standard since 2014.

Yet, von der Leyen was subsequently elected with a narrow majority by the European Parliament, promised to argue for a better implementation and formalisation of the desired parliamentary process in the future and announced that lead candidates Timmermans and Vestager would become vice-presidents in her commission.

[30] On 30 March 2021, Timmermans said in a tweet after talking to Swedish environmental activist Greta Thunberg that "The Commission remains committed" to making the Common Agricultural Policy "fulfill the objectives" of the European Green Deal.

[37] After members of the Labour Party and GroenLinks voted to contest the election on a joint list, Timmermans declared his intention to become lead candidate (lijsttrekker) for the alliance and to succeed Mark Rutte as Prime Minister of the Netherlands.

[41] The 2023 Dutch general election was held on 22 November 2023, and GL–PvdA ultimately finished in second place, with 25 seats, behind Geert Wilders' anti-immigration Party for Freedom (PVV).

[42] Timmermans ruled out working with the PVV, remarking that "we will never form a coalition with parties that pretend that asylum seekers are the source of all misery".

[44] Along with Mirjam Bikker of the Christian Union, Timmermans carried forward a bill to impose harsher penalties in cases of hate crimes, including acts of violence or insults motivated by discrimination.