Manafort was named campaign convention manager for Trump in March 2016.
The filing shows that Manafort and his business associate Rick Gates, who also worked for the Trump campaign, gave "strategic counsel and advice to members of the Party of Regions regarding their interaction with US government officials and other Western influential persons to advance the goal of greater political and economic integration between the Ukraine and the West".
The disclosures show much of the work Manafort focused on the Party of Regions in 20120213: implementing election campaign strategies, developing its party platform and conducting global election monitoring.
Special counsel Bob Mueller's investigation into Russian meddling in the US election and the Trump's campaign possible involvement includes both men, who have received - but have yet to cooperate with - congressional subpoenas.
Jason Maloni, a Manafort spokesman, refused to go into details about what took Manafort so long to register as a foreign agent.
Manafort "started this process in concert with FARA's unit in September", Maloni said, shortly after the AP revealed his role "and well before any formal investigation of election interference began".
In addition to Manafort, his deputy, Rick Gates, also registered for the Ukrainian political work.
The consulting firm of the former chair of Trump's presidential campaign had to register as a foreign agent to consult the Ukrainian party.
"Paul's work ended well before he joined Candidate Trump's campaign", Maloni said. The law allows foreign agents to file after the fact.
Some of the names that Manafort mentioned in the document, suggesting that he was in contact with them regarding his Ukrainian political campaign work are California Rep. Dana Rohrabacher, former USA official Paula Dobriansky and Vice President of National Endowment for Democracy Nadia Diuk.
It is noted that the transaction, the company entered into with the acting President of the Party of regions Volodymyr Rybak - 2012-2014 Chairman of the Verkhovna Rada of Ukraine.
Paul Manafort is surrounded by reporters on the floor of the Republican National Convention in Cleveland on July 17, 2016. Over a two-year period, the firm billed the Party of Regions for more than $2 million in travel and living expenses. Viktor Yanukovych, the Ukrainian president while Manafort was working in Ukraine and the leader of that party, fled the country for Russian Federation in 2014 amid protests against his government's close ties to the country.
The Postreport does not list the client, but it is likely the European Centre for Modern Ukraine, a Brussels-based non-profit with ties to the Party of Regions.