The attacker, who was caught assaulting the woman in Holloway, north London, was jailed for six years by Blackfriars Crown Court.
The case went ahead under the assumed name Aurilien Cheyroux, as the authorities could not confirm his true identity, other than him being French.
The judge ruled he should be deported once his sentence was served.
Passers-by saw "Cheyroux" partially-dressed and struggling with a woman on Caledonian Road in the early hours of 1 March, the court heard.
He ran off, but was caught by witnesses and held until police arrived.
The woman told the witnesses she was uninjured and left the scene before police arrived.
The attacker told police he was a French national and gave the name used in the trial.
But after taking finger prints the French authorities said the information was incorrect. They were unable to establish a true identity.
Det Const Kip Malek said: "This is a highly unusual case where we have been unable to trace the victim and the man who has been convicted is under an assumed name.
"Despite this we have been able to get a conviction and that is due to the brave members of the public who witnessed the attack and took it upon themselves to pursue this man and contact us."
Cheyroux was found guilty of attempted rape but had previously pleading guilty to sexual assault.
Det Insp Neil Smithson added: "I am still keen to ascertain his identity. Equally, if you were the victim of this offence please contact us."

The home affairs minister said he was refused a visa as the constitution prohibits hate speech.
Mr Anderson runs the Faithful Word Baptist Church, which says that homosexuality is an abomination punishable by the death penalty.
He said that religious freedom no longer existed in South Africa.
"I feel sorry for people who live in South Africa, but thank God we still have a wide open door in Botswana," Mr Anderson posted on his Facebook page after the decision to refuse him a visa.
South Africa has one of the most liberal constitutions in the world which protects gay rights.
The BBC's Milton Nkosi in Johannesburg says Mr Anderson was invited by a local church to visit this month.
Two petitions with more than 60,000 signatures, organised by gay and human rights activists in South Africa, had called for the trip to be stopped.
"I have identified Steven Anderson as an undesirable person to travel to South Africa," Home Affairs Minister Malusi Gigaba said.
Mr Anderson's church is based in the US state of Arizona and describes itself as an "old-fashioned, independent, fundamental, King James Bible only, soul-winning Baptist church".
In a video posted on YouTube before South Africa's decision was made, he said that the trip was not going to be about homosexuality, but he stood by his views against homosexuality, which he said were endorsed by the Bible.

The heart of the design shows a group of soldiers lifting a comrade, with his outstretched arms hinting at the crucifixion.
The artist, Nicholas Mynheer, said this represents "an act of compassion and service to others".
The window will be dedicated at a service on 10 July.
Mr Mynheer said it was a privilege to produce a window for "such a splendid building as Southwell Minster" and to spend time in his life entirely dedicated to considering the Great War.
"It was to be a truly humbling experience as I started to get an inkling of the stupendous sacrifice by all involved," he said.
"Initially, I felt overwhelmed by the accounts of sheer horror - mind numbing cruelty - and yet ever so gradually, amidst the darkness, little glimmers of hope appeared; of one man helping another, love overcoming hatred, and it was this that I realised I had to depict in the design."
Southwell Minster is in Nottinghamshire, but it was the mother church of Derbyshire and Nottinghamshire at the time of the war.
Mr Mynheer said the sacrifice was not just made in the trenches and at the war front, so those at home are also reflected in the window.
Beneath the central group of soldiers a woman hangs up washing, blown by the gathering storm.
This represents the women and others who remained at home, but also Mary at the foot of the cross.
A man has been jailed for attempted rape despite police not knowing who he is or finding his victim.
[NEXT_CONCEPT]
South Africa has barred controversial US pastor Steven Anderson from visiting the county because of his critical remarks about homosexuality.
[NEXT_CONCEPT]
A window commemorating the sacrifice of people from Nottinghamshire and Derbyshire in the First World War has been installed at Southwell Minster.