"Sarah kept mentioning this plan. She was nagging -- she wouldn't give it up. She would say, 'So, are you gonna let me adopt him?' We both kept telling her we were definitely not going to let her adopt the baby," Mr Johnston said.
"I think Sarah wanted to make Bristol look good, and she didn't want people to know that her 17-year-old daughter was going to have a kid."
In an interview with Vanity Fair, Mr Johnston (19) also claimed Mrs Palin was the supermom who did not cook, the hockey mom who almost never attended her son's hockey matches and the fearless outdoors-woman who had never touched a fishing rod.
Mr Johnston's comments come after his split from Mrs Palin's daughter, Bristol, who gave birth to their son Tripp in December.
It is the first time that the core personal qualities which so endeared Palin to the Republican base, have been questioned.
Exhausted, or bored, by her duties as head of Alaska's state government, Mrs Palin would return home most days by 5pm to take long baths and watch home improvement shows on television, Mr Johnston said.
Mrs Palin argued frequently with her husband about the idea of divorce and rapidly lost what enthusiasm she had for the Governor's job on her return to Alaska last November, he claims.
Most woundingly for a woman portrayed by the Republican campaign as a doting supermom, the former high school ice hockey star writes: "There wasn't much parenting in that house. Sarah doesn't cook, Todd [her husband] doesn't cook -- the kids would do it all themselves: cook, clean, do the laundry and get ready for school."
Given Mrs Palin's workload, voters might reasonably forgive her for cutting back on household chores but her political persona remains so closely aligned to what her supporters hope is the real woman underneath that such claims could damage her.
Mr Johnston's account describes being summoned without warning from work in the snowfields of northern Alaska to join the family at the Republican convention.
He claims to have hated it. Installed in a Minnesota hotel in which an entire floor was reserved for the Palins, he was sized up for new suits by Burberry and Armani.
"I remember thinking, 'How could this get any worse?'," he writes.
Mr Johnston has since complained of having access to his son restricted by the Palins.
His portrait of the family that might have moved into the vice-presidential mansion in the grounds of the US naval observatory in Washington is beyond dysfunctional. While Mrs Palin complained of the stresses of the Governor's job, her husband, a professional snow- mobiler, "was always out in the garage working on his snow machines and drinking beer or screwing off [loafing around]", Mr Johnston said.
The Palins appeared happily married for the cameras on the campaign trail, but slept in separate bedrooms during the Republican convention and "wouldn't go anywhere together unless the cameras were out".
When Mr Johnston learned that she had a gun and asked what type it was, she did not know, he claims. "It was in a box under her bed."
Meghan Stapleton, Mrs Palin's spokeswoman, did not immediately respond to request for comment, but has previously discounted Mr Johnston's allegations.
Outspoken Mrs Palin has been keeping a low profile since she resigned as Alaska's governor on July 26 with more than a year left in her first term. She is said to be writing her memoirs.
A week after Republican presidential candidate John McCain named Mrs Palin as his running mate, the campaign issued a statement that her unwed daughter was pregnant. It also said Bristol and the young man would marry.
Mr Johnston lived in the house for two months, but the couple called off the wedding shortly after the birth.