The 26-year-old's plea came on the eve of what would have been the first trial in a federal probe into the recruiting of US fighters for al-Shabab.
Mohamed faces up to 15 years in jail when sentenced later.
His lawyer, Peter Wold, said his client chose to admit the charge of providing material support to terrorists because he has a family and faced a much longer sentence if convicted.
Chief U.S. District Judge Michael Davis allowed Mohamed to remain free on electronic monitoring.
Others charged In a statement posted on the FBI's website, the US Attorney's office said Mohamed had admitted being a member of a conspiracy that recruited young men of Somali descent to travel to Somalia to fight Ethiopian troops. At least 21 are believed to have travelled from the US to Somalia.
End Quote B. Todd Jones US AttorneyParents were left to fret over the disappearance of their young sons, who often left home without a word”
"I helped them get tickets," he said.
Mr Wold said his client was motivated by patriotism and wanted to help others defend his homeland from "mortal enemies".
"[Mohamed] was only involved in a mission to protect Somalia," he said. "Omer has nothing to do with terrorists."
'Families torn apart' In a statement, US Attorney B. Todd Jones said: "Those involved in this conspiracy, including Omer Abdi Mohamed, violated the law in a dangerous and misguided effort to support a terrorist organisation.
"In the process, they tore apart many Somali-American families. Parents were left to fret over the disappearance of their young sons, who often left home without a word."
The US Attorney's office said Mohamed's guilty plea was the sixth in connection with the FBI's Operation Rhino investigation into the recruitment to al-Shabab.
Three men - Kamal Said Hassan, Abdifatah Yusuf Isse and Salah Osman Ahmed - have been convicted of terrorism offences after returning to the US from Somalia. Two others - Adarus Abdulle Ali and Abdow Munye Abdow - have been found guilty of obstruction offences.
Associated Press said plea bargains meant evidence from the previous cases had been kept mostly under wraps.
The US Attorney's Office says another man, Mohamud Said Omar, is in custody in the Netherlands awaiting extradition to faces charges relating to Operation Rhino and Ahmed Hussein Mahamud is being held in the US pending trial.
The FBI believes the earliest groups of US men recruited to al-Shabab left the US in October 2007.
It says US men took part in an ambush of Ethiopian troops in July 2008 and in October of that year Minnesota man Shirwa Ahmed took part in a suicide bombing. In June 2011, Farah Mohamed Beledi - also from Minnesota - died in an attempted suicide bombing at a checkpoint in Somalia.
The US government lists al-Shabab as a terrorist organisation. The al-Qaeda-linked group and its allies control much of Somalia's south. The country has had no functioning government since 1991.