Manjit Basuta, 44, faces possible life in prison when she is sentenced next month.
As the jury returned a unanimous verdict, she broke down and wept.
Her lawyers were eager to secure bail until sentence is passed on the 13 July, but the judge ruled that Mrs Basuta should be taken into custody immediately.
A spokeswoman for the San Diego prosecutor said: "She'll be behind bars for the rest of her life."
'Shaken baby syndrome'
The court was told that she grabbed 13-month-old Oliver Smith in a fit of temper when he refused to stop watching television to let her change his nappy.
She was then said to have shaken him violently and put him down hard. Oliver was taken to hospital, but died the next day.
The incident happened on 17 March last year at Mrs Basuta's San Diego home where she ran a day-care centre.
The autopsy showed bleeding and swelling in the baby's skull, and experts attributed his death to Shaken Baby Syndrome.
Mrs Basuta, an Indian-born British citizen, denied assaulting Oliver and causing his death.
'Previous head injury'
Her lawyers said the child had suffered a head injury some months prior to the incident and died after bumping his head while playing.
But Mrs Basuta's housekeeper, a key prosecution witness, testified that she saw her employer shake Oliver "very badly".
San Diego Superior Court Judge William Kennedy remanded her in custody for sentencing on 13 July.
The case has been compared to the murder trial in Boston of teenage British au pair Louise Woodward, who was convicted in October 1997 of murdering an 18-month-old baby.
The presiding judge then reduced the verdict to manslaughter and sentenced her to the 279 days she had already served in jail.
Country profiles
Safe nanny advice
(06 Apr 99 | UK)
Woodward lawyers to pursue new theory
(11 Mar 99 | UK)
Lawyers lash Woodward theory
(08 Mar 99 | UK)
Shaken baby syndrome
(27 Jan 99 | Medical notes)
Killer nanny escapes jail
(02 Feb 99 | UK)
San Diego Court
San Diego Public Law Library
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