He went into business. He became an executive for Royal Crown Cola. Glenn was friends with Robert F. Kennedy. Kennedy, an Attorney General of the United States and US Senator, told Glenn he should run for office. He planned on running for Senate in 1964, but an accident at home made him decide not to. In 1974, he ran for Senate as a Democrat. He defeated Senator Howard Metzenbaum in the primary. Glenn later beat Ralph J. Perk in the general election. He was reelected to the Senate in 1980. In 1984, he ran for President but the Democratic nomination went to Walter Mondale. Glenn was a popular senator and was re-elected several more times.