With his retirement, effective at the end of the month, he joins eight of his colleagues who have announced they will not run for re-election next year.
"I've tried my best to do it right. I've worked hard. I've delivered for my community, and I've made them proud. And I want to leave in a way that people can appreciate," he said outside the City Hall chambers.
Smith said his decision had nothing to do with next spring's election, saying he believes he would easily win re-election.
"My numbers are almost as high now as they were when I came in. Because I have done what my people have wanted me to do and I've been accessible. I've done the right thing. There was no question about me being re-elected. If I wanted to run tomorrow, my people would rally behind me," he said.
He did not specify a reason as to why he won't finish out his term or the urgency of his departure.
"I'm ready to go. Now. I want to go now," he said.
The longest-serving black alderman, Smith won his seat in the same election that gave Chicago its first African-American mayor, Harold Washington. His victory in 1983 was preceded by four failed attempts at elected office.
Once in office, however, he became an effective leader and remained free of scandal.
As chairman of the Health Committee, he was a major sponsor of Chicago's city-wide smoking ban.
Smith's open seat, as well as that of Ald. Toni Preckwinkle (4th), who is preparing to take over as Cook County Board president, and Ald. Tom Allen (38th), who was recently earned an appointment as a Circuit Court Judge, must be filled by the mayor with appointments within 60 days of their retirements.