NEW YORK— Steven Donziger once stood to gain hundreds of millions of dollars for winning a $9.5 billion environmental-contamination verdict against Chevron Corp. CVX -0.41% in Ecuador.

Now, that judgment has been discredited, Mr. Donziger has lost his New York law license, and he is on trial for criminal charges that he flouted a judge’s orders.

The contempt-of-court trial that started Monday before U.S. District Judge Loretta Preska is the latest twist in a legal saga involving Chevron and Mr. Donziger that has lasted nearly three decades and spanned the globe.

“Steven Donziger…has intentionally and repeatedly disobeyed court order after court order after court order,” Rita Glavin, a private lawyer serving as the prosecutor, said during opening statements Monday.

Martin Garbus, an attorney for Mr. Donziger, said in his opening remarks that his client “is innocent of every single charge.” He said the court had greenlighted an intimidation campaign by Chevron against Mr. Donziger.

“Mr. Donziger takes his responsibility to his clients, the people of Ecuador” seriously, Mr. Garbus said.

Mr. Donziger first sued Chevron predecessor Texaco Inc. in New York in 1993 on behalf of native Ecuadoreans who alleged the company’s oil operations in the Amazon were sickening them. The dispute later moved to Ecuador, where a judge in 2011 issued the $9.5 billion judgment against Chevron.

The San Ramon, Calif.-based oil company has spent the past decade punching back, securing a U.S. court decision finding that Mr. Donziger and his colleagues corrupted the legal process in Ecuador by manufacturing evidence, pressuring judges and ultimately ghost writing the final judgment in the case.

Those conclusions came in a 2014 ruling by U.S. District Judge Lewis Kaplan that also ordered Mr. Donziger to relinquish his 6.3% contingency fee interest in the case to Chevron and forbade him from profiting from the Ecuador judgment or trying to enforce it in the U.S. Mr. Donziger denies he acted fraudulently in Ecuador.