Siekierski served as Secretary of State for Agriculture in Warsaw from 2001 to 2003, and since Poland's accession to the European Union in 2004, he has been a member of the European Parliament. From 2014 to 2019, he chaired the Committee on Agriculture and Rural Development there. Siekierski is a member of the Polish Peasant Party (PSL). In 2019, he returned to Polish politics and joined the three-party coalition founded by former EU President Tusk against the conservative PiS party government.
The new prime minister described the Siekierski-Kołodziejczak duo as "promising": "Polish farmers, residents of Polish villages, will surely see what a tough, competent fight for their interests means," Tusk argued, pointing out that Kołodziejczak was among the first to warn about the consequences of the influx of Ukrainian grain across Polish territory.
As his first act as deputy minister, Michał Kołodziejczak submitted an interpellation regarding the labeling of food products in Polish retail chains. Kołodziejczak claims that retailers mislead consumers by applying different prices on labels. He believes that for each product, the amount that goes to the Polish farmer ('producer') should be indicated.