Weiner agreed Friday not to appeal any sentence between 21 and 27 months in prison. He was forced to surrender his iPhone.
Abedin was not in court Friday, when Weiner told the judge that for years he had "destructive impulses" that ruined his family life and crushed his dreams.
Weiner, a Democrat, was married to top Clinton aide Huma Abedin at the time he began conversing with the Gaston teen. In a case of adding insult to injury, she filed for divorce on the same day as her husband's guilty plea.
He must register as a sex offender and faces a possible prison term.
May 22: Weiner launches his re-entry into the world of politics with a run for New York City mayor, asking voters for a second chance. Weiner's sentencing is scheduled for September 8.
USA media reported shortly after the guilty plea that wife Huma Abedin had filed for divorce. Abedin, the former assistant and chief of staff for Hillary Clinton, previously separated from Weiner in August 2016, five years after it was initially reported that he sent sexually driven texts to other women.
Weiner was already in federal custody ahead of the hearing, which lasted less than half an hour.
In late October, just days before the election, FBI director James Comey stunned the country by announcing that his agency was reopening its closed investigation into Clinton's handling of State Department business on a private email server so it could analyze the newly discovered correspondence. After reopening the probe, the Federal Bureau of Investigation determined the emails on Weiner's computer did not change the agency's decision that no charges should be brought against Clinton.
Clinton believes the hit her campaign took from the revelation led her to lose the election to Donald Trump.
Following the incident with the 15-year-old girl, Weiner said, "I came to grips for the first time with the depths of my sickness" and started mental health treatment.
Weiner was forced to resign his seat in Congress in 2011 after a similar scandal in which he admitted to exchanging sexually explicit pictures and messages with at least six women. His failed mayoral bid was the subject of the documentary "Weiner".
The New York Post reported that Weiner had sent sexual messages along with the alleged photo to an unidentified woman in 2015.