Reiner has devoted considerable time and energy to liberal activism.
His lobbying as an anti-smoking advocate, in particular, prompted his likeness to be used in a satirical role in a South Park episode titled "Butt Out".

Reiner is a co-founder of the American Foundation for Equal Rights, which initiated the court challenge against California Proposition 8 that banned same-sex marriage in the state.

In 1998, Reiner chaired the campaign to pass Prop 10, the California Children and Families Initiative, which created First 5 California, a program of early childhood development services, funded by a tax on tobacco products.
He served as the first chairman of First 5 California, from 1999 to 2006.
Reiner came under criticism for campaigning for a ballot measure (Prop 82) to fund state-run preschools while still chair of the First Five Commission, causing him to resign from his position on March 29, 2006.
An audit was conducted, and it concluded that the state commission did not violate state law and that it had clear legal authority to conduct its public advertising campaigns related to preschool.
Prop 82 failed to win approval, garnering only 39.1% support.

Reiner is a member of the Social Responsibility Task Force, an organization advocating moderation where social issues (such as violence and tobacco use) and the entertainment industry meet.
He is also active in environmental issues, and he successfully led the effort to establish California's Ahmanson Ranch as a state park and wildlife refuge rather than as a commercial real estate development.
He introduced Spinal Tap at the London Live Earth concert in July 2007.

Reiner was mentioned as a possible candidate to run against California Governor Arnold Schwarzenegger in 2006 but decided not to run for personal reasons.
He campaigned extensively for Democratic presidential nominee Al Gore in the 2000 presidential election, and he campaigned in Iowa for Democratic presidential candidate Howard Dean just before the 2004 Iowa caucuses.
He endorsed Hillary Clinton for president for the 2008 election.
In 2015, he donated US$10,000 to Correct the Record, a political action committee that supported Hillary Clinton's 2016 presidential campaign.
Since the 2016 election, he has continued to campaign against Donald Trump, calling him a racist, sexist, anti-gay, and anti-Semitic, and he compared him to the Nazi police at Auschwitz.
Reiner said that Harvey Weinstein is a "bad guy" but Trump is "also an abuser".

Reiner serves on the Advisory Board of the Committee to Investigate Russia.

Reiner endorsed Joe Biden for president for the 2020 election.
In March 2020, Reiner demanded that President Trump be removed from office due to what he considers Trump's poor handling of the COVID-19 pandemic.
Reiner has devoted considerable time and energy to liberal activism. His lobbying as an anti-smoking advocate, in particular, prompted his likeness to be used in a satirical role in a South Park episode titled "Butt Out".
Reiner is a co-founder of the American Foundation for Equal Rights, which initiated the court challenge against California Proposition 8 that banned same-sex marriage in the state.
In 1998, Reiner chaired the campaign to pass Prop 10, the California Children and Families Initiative, which created First 5 California, a program of early childhood development services, funded by a tax on tobacco products. He served as the first chairman of First 5 California, from 1999 to 2006. Reiner came under criticism for campaigning for a ballot measure (Prop 82) to fund state-run preschools while still chair of the First Five Commission, causing him to resign from his position on March 29, 2006. An audit was conducted, and it concluded that the state commission did not violate state law and that it had clear legal authority to conduct its public advertising campaigns related to preschool. Prop 82 failed to win approval, garnering only 39.1% support.
Reiner is a member of the Social Responsibility Task Force, an organization advocating moderation where social issues (such as violence and tobacco use) and the entertainment industry meet. He is also active in environmental issues, and he successfully led the effort to establish California's Ahmanson Ranch as a state park and wildlife refuge rather than as a commercial real estate development. He introduced Spinal Tap at the London Live Earth concert in July 2007.
Reiner was mentioned as a possible candidate to run against California Governor Arnold Schwarzenegger in 2006 but decided not to run for personal reasons. He campaigned extensively for Democratic presidential nominee Al Gore in the 2000 presidential election, and he campaigned in Iowa for Democratic presidential candidate Howard Dean just before the 2004 Iowa caucuses. He endorsed Hillary Clinton for president for the 2008 election. In 2015, he donated US$10,000 to Correct the Record, a political action committee that supported Hillary Clinton's 2016 presidential campaign. Since the 2016 election, he has continued to campaign against Donald Trump, calling him a racist, sexist, anti-gay, and anti-Semitic, and he compared him to the Nazi police at Auschwitz. Reiner said that Harvey Weinstein is a "bad guy" but Trump is "also an abuser". 
Reiner serves on the Advisory Board of the Committee to Investigate Russia.
Reiner endorsed Joe Biden for president for the 2020 election. In March 2020, Reiner demanded that President Trump be removed from office due to what he considers Trump's poor handling of the COVID-19 pandemic.
Rob Reiner