WASHINGTON—Paul Manafort, President Trump’s former campaign chairman, acknowledged new details on Friday about his work on behalf of a former Ukrainian president—and his efforts to keep that work secret—as he pleaded guilty to criminal charges arising from special counsel Robert Mueller’s investigation.

In court documents associated with his guilty plea, Mr. Manafort said his work for Viktor Yanukovych, the Ukrainian president who was ousted before fleeing to Russia in 2014, included “an all-out campaign to try to kill” congressional efforts to criticize his client.

A key part of Mr. Manafort’s effort, according to the documents, was working with former European leaders to lobby U.S. policy makers. The prominent Europeans, whom Mr. Manafort referred to as the Hapsburg Group, lobbied members of Congress and the administration without disclosing that they were being paid by Ukraine.

In a June 2012 “eyes only” memo, Mr. Manafort said the goal was to “assemble a small group of high-level” European officials who could act “informally and without any visible relationship with the government of Ukraine.”

The group included a former Austrian chancellor, Italian prime minister, and Polish president. In memos to Mr. Yanukovych included in the court documents Friday, Mr. Manafort mentions having “organized and leveraged” visits to Washington by former Polish President Aleksander Kwasniewski and former Italian Prime Minister Roman Prodi, who he says also signed op-eds casting Ukraine as an “important global strategic partner to the U.S.” Mr. Manafort also makes repeated references to a third member of the group called “AG,” an apparent reference to former Austrian Chancellor Alfred Gusenbauer.

The plea agreement says Mr. Manafort arranged for the former European officials to be paid more than €2 million ($2.3 million) from 2012 to 2014.

In the fall of 2012, at Mr. Manafort’s behest, members of the Hapsburg Group contacted U.S. senators to lobby them to stall a resolution critical of Mr. Yanukovych’s treatment of former Prime Minister Yulia Tymoshenko, his political rival. The effort ultimately failed, and the Senate approved the resolution.

Starting in 2006, Mr. Manafort and his associate Richard Gates—who pleaded guilty earlier this year to charges related to the Ukraine work and was a witness for prosecutors in Mr. Manafort’s first trial—engaged in a multimillion-dollar lobbying campaign for Mr. Yanukovych, who was seeking to burnish his image as a Europe-oriented leader despite close ties to Moscow.

To help with their Ukraine work, Mr. Manafort hired a half-dozen lobbying and law firms in Washington, including the Podesta Group, Mercury LLC and Skadden, Arps, Slate, Meagher & Flom LLP. The court documents say the firms hired by Mr. Manafort were paid a total of at least $11 million for their efforts.

Mr. Manafort took measures to conceal his client’s role in the lobbying efforts. “It is very important we have no connection,” he told his lobbyists in 2013, according to the documents. He told the Washington lobbying firms he hired to report that the lobbying work was being funded and directed by the European Centre for a Modern Ukraine, which the documents note “was not even operational” when the companies began lobbying for Ukraine.

Employees at the firms acknowledged privately whom they were working for, the court documents indicate. The head of Podesta Group—the company’s founder, Tony Podesta, who isn’t named in the documents—advised his team to “think the President of Ukraine ‘is the client,’” the documents say, while another Podesta Group employee referred to the Centre in an email as the “European hot dog stand for a Modern Ukraine.” A Mercury employee told another company employee that the lobbying for the Centre was “in name only” and added: “You’ve gotta see through the nonsense of that…It’s like Alice in Wonderland.”

Mr. Mueller referred investigations related to those firms to the U.S. attorney’s office in Manhattan earlier this year, according to a person familiar with the matter. A former Skadden associate was prosecuted by Mr. Mueller’s office for lying to investigators and served a one-month sentence in jail. No Podesta or Mercury associates have been publicly charged.

Representatives for Skadden, Mercury and the Podesta Group, which folded last year, haven’t responded to requests for comment.

The documents also name Mr. Manafort’s Ukrainian business associate, Konstantin Kilimnik, who was previously charged with conspiring with Mr. Manafort to obstruct justice by trying to sway the testimony of two potential witnesses in the Mueller probe. Mr. Kilimnik isn’t in custody and is believed to be overseas.

Messrs. Manafort and Kilimnik made 21 contacts or attempted contacts with the witnesses between late February and early April this year, according to the court documents filed Friday.

A few weeks before the 2016 election, Mr. Gates told another business associate, Alex van der Zwaan, that Mr. Kilimnik was “a former Russian intelligence officer,” according to a previous court memo.

—Brody Mullins contributed to this article.

Write to Rebecca Ballhaus at Rebecca.Ballhaus@wsj.com and Julie Bykowicz at julie.bykowicz@wsj.com