He specialized in civil law. He did anti-corruption work. He uncovered evidence of tax fraud. He implicated police, judiciary figures, tax officials, bankers, and Russia's mafia.
He accused them of stealing around $230 million dollars in 2007 through fraudulent tax refunds.
On issues relating to courts, taxes, fines, and civil law, he was called the "go to guy" in Moscow.
In November 2008, he was arrested, imprisoned, and treated abusively. For 11 months he was denied family visits. Serious health problems developed. Inadequate treatment followed.
On November 16, 2009, he died for reasons attributed officially to a "rupture to the abdominal membrane" and subsequent heart attack. If trial proceedings didn't begin, he was due to be released eight days later.
His death sparked public outrage. Improving prison healthcare was demanded. Reducing the number of inmates awaiting trial was stressed.
In December 2009, an independent Moscow Public Oversight Commission said he was subjected to "psychological and physical pressure...."
Initially his death was blamed on medical neglect. Later claims suggested murder. Official investigations began. In July 2011, death by medical neglect was ruled.
The 2012 Sergei Magnitsky Rule of Law Accountability Act normalized US/Russian trade relations. Doing so came with strings. Moscow raised legitimate objections.
The legislation imposes visa bans, asset freezes, and other sanctions on Russian nationals accused of committing human rights abuses.
It mandates publishing and updating information on individuals the Secretary of State believes were responsible for detaining, abusing, and/or causing Magnitsky's death.
Others accused of concealing what happened are targeted. So is anyone believed to have benefitted financially, as well as those involved in an alleged criminal conspiracy regarding his treatment and death.
Targeted offenses include extrajudicial killing, torture, and/or other human rights violations committed against individuals seeking to expose illegal Russian activity, or against persons promoting human rights and freedoms.
On December 21, Russia's State Duma lower house retaliated. It passed the Dima Yakovlev bill.
On December 26, Russia's upper house Federation Council followed suit. It passed the measure unanimously. It imposes visa bans and asset freezes on US officials accused of violating the rights of Russian citizens abroad.
It prohibits US-sponsored NGOs from operating in Russia disruptively. It also targets US citizens associated with them. Another provision bans US citizens from adopting Russian orphans.