GREENSBORO, N.C. — Testimony in the federal corruption trial of former Senator John Edwards began Monday with the man on whom much of the government’s case hinges, Andrew Young, a former campaign staff member once so close to Mr. Edwards that he claimed paternity of the child Mr. Edwards had tried to hide.

David V. Harbach II, who is directing the prosecution team, walked the jury through Mr. Young’s relationship with the Edwards family and his connections to Rachel Mellon, the elderly banking heiress who contributed $725,000 of the money at the heart of the case and said she would do anything to support Mr. Edwards’s presidential campaign.

Ms. Mellon was reportedly infuriated by criticism that Mr. Edwards received for getting expensive haircuts. “From now on, all haircuts, etc., that are necessary and important for his campaign — please send the bills to me — it’s a way to help our friend without government restrictions,” she wrote in a note to Mr. Young.

Mr. Young, who has immunity from prosecution, is expected to be on the stand all day Tuesday and probably into Wednesday as the government tries to prove that Mr. Edwards, a two-time presidential hopeful, clearly knew that nearly $1 million from Ms. Mellon and another wealthy donor were illegal campaign contributions and not, as he has claimed, aid from friends trying to help him hide an affair from his wife, Elizabeth Edwards. Mrs. Edwards died in 2010 after a public battle with breast cancer.