Chloe Thomas, 25, originally denied neglecting Finley, who was found with "catastrophic" injuries at their home near Tonypandy in September 2014.
She changed her plea and was sentenced at Cardiff Crown Court on Monday.
Her boyfriend Sean Buckley, 28, was jailed for life after being found guilty of child cruelty and murder.
Buckley had claimed Finley fell down the stairs but he was found to have carried out a "deliberate" attack on him that included hitting him with a garden chair.
The toddler died after suffering a catalogue of injuries that included a fractured skull and broken ribs.
When he was sentenced in June, a judge told Buckley he must serve a minimum 17 years but said he may never be released.
In mitigation, Sally Howes said that while Thomas admitted neglect and allowing cruelty, she did not carry out an assault herself.
The defendant was described as being of "low intellectual capacity" and "a proud mother" before she started a relationship with Buckley.
A post-mortem examination found Finley also had illegal drugs in his system and while Ms Howes said Thomas did not administer these, she admitted exposure to them was "unacceptable by anyone's standards".
She asked for a custodial sentence to be suspended because Thomas was suffering from a "delayed grief reaction".
But, in handing her a 20-month term, Judge Frances Patterson said: "You should have been caring for your son and not closed your eyes to his deteriorating condition."
Head of NSPCC Cymru Des Mannion criticised Thomas for failing to protect her son from the "horrific abuse he suffered at the hands of Sean Buckley".
He said: "Everyone who followed this case would have been deeply affected by hearing the evidence of brutality that blighted Finley's life and it is right that his mother has been jailed for her part."
Mr Mannion said he hoped a detailed review of the case would help provide answers about what had happened.
A mother who admitted causing or allowing cruelty towards her 17-month-old son who was later murdered has been jailed for 20 months.