Born in 1962 to a Turkmen family, Önder was shaped by diverse ideological influences: his father was a socialist and a founding member of the Workers' Party of Turkey (TİP) in Adıyaman, while his uncle was a follower of Said Nursi. Önder lost his father at the age of 8.

In 1976, at just 14, he encountered Abdullah Öcalan in Adıyaman — a meeting that sparked his lifelong engagement with the Kurdish freedom movement. During his youth, he participated in socialist activism and was first arrested in 1978 while protesting the Maraş Massacre. After the 1980 coup, he was imprisoned and tortured, spending seven years in Mamak Prison.

Following his release, Önder worked in various jobs in Istanbul and briefly lived abroad. He continued supporting Kurdish rights and contributed articles promoting the free use of the Kurdish language.

In the 2000s, he transitioned into film and literature. His 2006 film Beynelmilel, which he co-directed and co-wrote, earned national acclaim. He also wrote O... Çocukları (2008) and contributed to F Tipi Film (2012).

After 2010, he wrote for newspapers including BirGün, Radikal, and Özgür Gündem. In 2011, he entered parliament as an independent and later joined the Peoples’ Democratic Party (HDP), where he served in senior roles.

During the 2013 Gezi Park protests, Önder gained national attention by physically standing in front of bulldozers to protect trees. He was later hospitalized after being struck by a tear gas canister.

From 2013 to 2015, he played a leading role in Turkey’s “Solution Process,” conducting negotiations with Abdullah Öcalan and Kurdish leaders. He read Öcalan’s Newroz message in 2013 in Diyarbakır, a symbolic moment in the peace process.

He ran for mayor of Istanbul in 2014 and was re-elected as an MP in 2015 and again in 2023. He was arrested in 2018 on charges of “terrorist propaganda” but released in 2019. Until his death, he served as Deputy Speaker of Parliament.