A veteran Republican activist whose quest to obtain Hillary Clinton’s emails from hackers dominated the final months of his life struck up a professional relationship with Lt. Gen. Michael Flynn, the former national security adviser to President Trump, as early as 2015, and told associates during the presidential campaign that he was using the retired general’s connections to help him on the email project.

The late Peter W. Smith, an Illinois financier with a long history in Republican politics, met with Mr. Flynn in 2015, according to people familiar with the matter. At the time, Mr. Flynn had recently left his job as head of the Defense Intelligence Agency and was trying to set up his own consulting firm, while Mr. Smith was looking at investment opportunities in cybersecurity.

Mr. Flynn was a top adviser to Mr. Trump during the 2016 campaign. He served briefly as national security adviser in Mr. Trump’s administration before being forced to resign and pleading guilty to a charge of lying to authorities about his conversations with a Russian ambassador. He is scheduled to be sentenced in December.

Additionally, in an email reviewed by The Wall Street Journal, one of Mr. Smith’s former associates wrote to a friend last week, “As you are aware Peter started a business relationship with Gen. Mike Flynn in November 2015. We spoke with him on the day he left for his trip to Moscow.” The associate, John Szobocsan, sent the email as the Journal was preparing a story on Mr. Smith and was attempting to reach Mr. Szobocsan. He didn’t respond to requests for comment.

The Journal reported in 2017 that Mr. Smith implied he had connections to Mr. Flynn, but the email and people familiar with the matter indicate the two men were in contact and did in fact have a working relationship. Though no apparent business deals came of the 2015 meeting, the introduction gave Mr. Smith a contact who would go on to become part of Mr. Trump’s inner circle. An attorney for Mr. Flynn declined to comment.