Sir Christopher Meyer, 74, was beaten by two youths and left with injuries to his eye, nose and lip.
His wife Baroness Catherine Meyer told The Times that Sir Christopher had been taking the Tube home when he was attacked.
She sad: “He looks terrible. His left eye is like a golf ball and bleeding, the nose looks like it could be broken.”
She said her husband did not remember the attack, adding: “The first thing he remembers is the police.
“At first I thought that the attack was politically motivated – he is opinionated and sometimes people have different opinions – but the police told me they believe that it is more likely that they might have wanted to rob him.”
Nothing was taken, however.
British Transport Police said a 16-year-old boy and a 15-year-old girl have been arrested on suspicion of assault occasioning grievous bodily harm. They have been released under investigation while inquiries continue.
A photo published in The Times showed Sir Christopher in a Paddington hospital bed with his eyes closed and his face covered in blood.
Sir Christopher joined the Foreign and Commonwealth Office in 1966 and served in a number of posts until his retirement in 2003.
He is best-known, however, as British ambassador to the US, a post he held from 1997 until he retired.
His time there coincided with the presidencies of Bill Clinton and George W Bush, and also included the 9/11 terrorist attacks and wars in Kosovo, Sierra Leone and Afghanistan.
During his career, he also spent two years as chief press secretary to prime minister John Major and acted as chief speech writer for three foreign secretaries – James Callaghan, Anthony Crosland and David Owen.
Between 2003 and 2009 he was chairman of the now defunct Press Complaints Commission.