David Harper, 36, began a relationship with married Jennifer Maguire before plotting to abuse the infant.
He began encouraging Maguire, 31, to take photographs and footage of herself sexually assaulting the child before sending them to him.
The pair began a relationship in 2012 while working at the Bank of Scotland in Uddingston, South Lanarkshire, and would have sex in the office when other colleagues were not there.
Their crimes were uncovered by bank bosses following an investigation into claims Harper, a self confessed sex-addict, had been harassing another female employee.
Yesterday at Hamilton Sheriff Court, Harper was jailed for four-and-a-half years and Maguire was ordered to be held for three years.
They both admitted a charge of sexual assault on a child while Harper, of Uddingston, also admitted making indecent photographs of children and threatening to publish private photographs of another woman.
Maguire, of Blantyre, admitted a charge of distributing or showing indecent photographs.
Fiscal depute Natalie Henderson said: “Jennifer Maguire committed the sexual acts and recorded them and sent them to David Harper. The accused Harper accepts that he invited her to do this and she agreed.
“Her position is that she took and sent the images because David Harper had threatened to put naked images of her on the internet. The accused Harper disputes this and says he said no such thing to her.”
Officers went to Harper’s home and confiscated his laptop where they found 9,870 images of children and videos.
Police also discovered Harper and Maguire had been using the WhatsApp messaging service to talk about children, which included Harper describing what he would do to them.
Defending Maguire, advocate Louise Arrol said: “There are unusual circumstances around the nature of the offending and I ask you take into account the catastrophic impact her offending has had on her.”
Defending Harper, advocate Lili Prais said: “He is attending Sex Addicts Anonymous and Stop It Now and both schemes are being attended voluntarily.”
Sheriff Daniel Kelly QC said: “It is clear that you, David Harper, were the instigator in what occurred during this case.
“I view you as a serious risk to women and you pose a serious risk of sexual harm to both women and children.
“You, Jennifer Maguire, are of medium risk of re-offending and have limited insight into your actions but you provided a detailed account to police and were prepared to give evidence against David Harper.”
Maguire will be supervised for one year on her release while Harper will be monitored for three. Both were both placed on the sex offenders register indefinitely.