He served as chairman of the Key of Croatia party (formerly known as Human Shield) and as a member of the European Parliament from 2019 to 2024.

[1] In 2016, he married fellow Human Shield activist and former party co-chairwoman Vladimira Palfi.

[citation needed] Along with former-fellow MP Ivan Pernar, Sinčić was one of the founders of Human Shield in 2011.

Sinčić was a candidate in the first round of the 2014 Croatian presidential election, coming in third with 16.42% of the vote.

[4] On 4 June 2019, the presidency of his party decided that Sinčić would still occupy his seat in the European Parliament because he won the most preferential votes in the elections held on 26 May 2019.