[4] His career in journalism began in 2000 after the probation at the news program Reporter on the Novyi Kanal channel chaired by Andriy Shevchenko.

In 2014, Leschenko was elected a people's deputy to the 8th Ukrainian Verkhovna Rada with the party Petro Poroshenko Bloc "Solidarity".

He chaired a subcommittee on International Partnership and Anticorruption Law Implementation as part of the Verkhovna Rada Committee on Corruption Prevention and Counteraction.

In June 2013, Leschenko published a documentary book the "American saga of Pavlo Lazarenko" based on the US law enforcement agencies' investigation materials.

[12][13] On 26 November 2015, Leschenko presented the documents that he believed had justified the criminal case in Switzerland on bribery and money laundering.

[14] In fall 2017, together with other people's deputies and civil society representatives, Leschenko held an agitation campaign in support of "Vseukrayinskyi zbir" initiative, which had its focus on "a large political reform".

The main points of the reform were a creation of an anticorruption court, abolition of deputies' immunity, a switch to a proportional representation election system with open lists.

Serhiy advocates for putting a limit on pre-electoral agitation campaigns on television, which will increase the candidates' ability to compete.

After election to the Verkhovna Rada, Leschenko gradually began to criticize the Petro Poroshenko Bloc (PPB) more and more and stopped voting in sync with it.

[21] On 6 September 2016, a group of deputies of the Verkhovna Rada appealed to the National Anti-Corruption Bureau (NABU) with a request to check the purchase of Serhiy Leschenko of an apartment of 192 m² in a new building on Ivan Franko Street in the center of Kyiv.

[24] According to Leshchenko, half of the amount was borrowed from the chief editor of the Ukrainska Pravda Olena Prytula, in the form of an interest-free cash loan for 10 years, the rest was personal funds of Leschenko and his girlfriend, famous Ukrainian DJ Anastasiya Topolska.

[22] The seller of this apartment was the Asset Management Company Industrial Investments LLC, which is indirectly related to the oligarch Dmytro Firtash.

In addition, USD 77 thousand were spent from the deposits of Serhiy Leschenko's mother, which wasn't stated in the deputy's declaration as his own funds.

[29] On 15 February 2017, Pechersky District Court of Kyiv closed an administrative case, not finding corpus delicti in Leschenko's actions.

[31][32][33] On 9 February 2017, people's deputy from the Petro Poroshenko Bloc Ivan Melnychuk started a verbal dispute with Serhiy Leschenko in the session hall of the Verkhovna Rada.

[34][35][36] In August 2016, Serhiy Leshchenko announced that the Party of Regions, shortly before Viktor Yanukovych's election as the President of Ukraine, paid to the American political consultant Paul Manafort for his services.

[46] On 10 May 2019, 107th Mayor of New York City and attorney to US President Donald Trump, Rudolf Giuliani, announced that he had hoped to meet in Kyiv with Ukrainian president-elect, Volodymyr Zelensky.

[18] Leshchenko said that the Prosecutor General, Yuriy Lutsenko, was to blame for the scandalous situation, he rejected the accusation of forging "black bookkeeping" materials.

[citation needed] On 18 December 2019 the Honcharuk Government appointed Leshchenko a member of the Supervisory Board of the Ukrainian Railways.