Lisa Passey, 28, and her former partner Wayne Dale, 45, left 13-month-old Kian Dale and a two-year-old child alone while the pair entertained a friend, a court heard.
Earlier, Passey told Worcester Crown Court she had been "stupid" to leave Kian unsupervised for 15 minutes.
Passey, from Tenbury Wells, and Dale, will be sentenced on 25 April.
They were also found guilty of child cruelty with wilful neglect.
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Kian Dale had been left in an upstairs bath seat in the couple's home in Kyreside, Tenbury Wells, on 26 September, 2015, Worcester Crown Court was told.
Passey was with a friend downstairs while Dale was "socialising" and "using his computer".
When he went upstairs to check on Kian he had drowned.
Prosecutors said Passey and her friend had been drinking coffee and smoking outside.
After Kian was discovered lifeless in the water, Passey dialled 999 telling the operator her baby had "drowned in the bath".
The court heard Dale, of no fixed address, told police officers he "had a beer, rolled a cigarette outside and burned a CD" and that he had only left his son for "a couple of minutes".
A post-mortem examination found the child's death was consistent with drowning, including what was believed to be soap bubbles in his lungs.
Ms Passey, of Tenbury Wells, initially claimed she had not run a bath, but twice changed her account, the prosecutor alleged.
The parents of a toddler who drowned in a bath have been found guilty of manslaughter.