Born and raised in Kuala Lumpur, Teresa is a second-generation Malaysian of Chinese descent. Kok is a member of the Hakka dialect group and her ancestors were from Huizhou, Guangdong Province, China. She speaks Malay, English and Chinese fluently. Kok is a Catholic by religion.
She earned her first degree from Universiti Sains Malaysia (USM), graduating in the School of Communication, and; a second degree from University Malaya in political science, and a master of philosophy. Her thesis was on United Malays National Organization (UMNO), titled "Factionalism in Umno During Dr Mahathir's Era (1981–2001)".
Kok writes in weekly column for Chinese daily Sin Chew Jit Poh. In 2004, she published a book compiling articles she wrote for the then Chinese daily.
Kok was political secretary to Opposition Leader Lim Kit Siang from 1990 to 1995. In 1995, she contested the Ipoh Barat Parliamentary seat on a DAP ticket but was defeated by the Malaysian Chinese Association (MCA) candidate. Kok resigned as political secretary after that to further her studies.
In the 1999 general election, Kok won the Parliamentary seat of Seputeh in Kuala Lumpur with a majority of 5,200 and was re-elected in 2004 with a majority of 12,895, the largest winning margin among the 13 elected DAP MPs.
In the 2008 general election, Kok retained the Seputeh seat with a majority of 36,492, the largest majority in any constituency and won the Kinrara seat in Selangor State Legislative Assembly at the same time. She was elected in the new Selangor executive council, and was named senior executive councillor who was put in charge of investment, trade and industry to ensure all funds are directed to Malaysians.
In 2013, she won re-election to Parliament, garnering over 86 percent of the formal votes cast in her Seputeh constituency.
Later in 2008, she was arrested under the Internal Security Act (ISA). Under the act, the police have no obligation to disclose the alleged offence if any at all. The Malay newspaper Utusan had reported she had "advised" a mosque in Puchong not to use loudspeakers while making the azan. She denied the allegation. It was found out later that a faulty loudspeaker system was the reason why the mosque did not broadcast the azan. Furthermore, while there was a petition sent to the mosque, the petition requested for the mosque to lower the volume during 'ceramah' or sermons and not during the azan. The administrator of the mosque, as well as the petitioners, confirmed that Kok was not involved in the petition. She was released on 19 September 2008. On 27 September, two Molotov cocktails were thrown into the compound of her family residence, accompanied by a warning letter. No-one was hurt.
In May 2014, Kok was charged with sedition for making a Chinese New Year video posted on YouTube which allegedly contained seditious elements. Kok was among the first of several other opposition politicians to be caught in a nationwide sedition dragnet.Kok, Teresa (2002). "Government Should Not Send The Rohingya Refugees Who Broke Into The UNHCR Office Back To Burma". Retrieved 5 November 2005.
Kok, Teresa (2005). "Teresa Kok Suh Sim". Retrieved 29 October 2005.